Until the 1960s, it was rare to find mention of Aquinas in non-Thomist English-language philosophy, and any reference would most likely be in connection with natural law ethics and natural theology. Anyone who did not know better could have formed the belief that these were the sole topics of Aquinas’s concern. In truth, however, he addresses more philosophical topics than most thinkers, and the texts standardly quoted in relation to ethics and philosophy of religion constitute a tiny (but important) part of his total corpus, which has been calculated to run to over eight and a half million words.
Following the publication in the late 1950s of writings by Peter Geach in which he drew upon Aquinas to illuminate issues in metaphysics, philosophy of mind and metaethics, and the appearance in 1961 of his long essay on Aquinas, things began to change. It became more common to see references to and, in time, discussions of Aquinas’s ideas about the nature of substance and causality, and of mind, knowledge and agency; as well as to find more wide-ranging discussions of his philosophical theology and moral, political and legal philosophy. This interest has grown, and there is now a significant number of books and essays in which Aquinas’s thought is examined in some detail. There are, however, still many aspects of his writings that remain unknown to those outside the field of Thomistic studies; or which, though vaguely known are generally misunderstood. These include issues which have been quite widely debated among followers and critics of Aquinas, and they number matters where Thomas’s own view is other than what might have been supposed. Examples of such matters include the nature of angels, the condition of disembodied souls, the extent of actual human knowledge of nature, and the origins of individual human life.
The last of these is the subject of a chapter in a recent book by Robert Pasnau on Aquinas and Human Nature, a work which itself is an example of the extended interest mentioned above. Since there will be readers whose only knowledge of the issues in question will come from Pasnau’s account, and since that account is contentious in substance, and advanced in advocacy of a particular moral interest, it is necessary to provide another, and, we believe, more credible account of the issue of when human life begins, as this may be determined on the basis of known empirical facts and Aquinas’s metaphysics, and a more accurate representation of how (and how extensively) this issue has been treated hitherto. Whatever readers may conclude about the substantive issue they will, we hope, see that matters are other than as Pasnau has chosen to present them.
Pasnau on Aquinas and others on Abortion
The subtitle of Pasnau’s book is ‘A Philosophical Study of Summa Theologiae Ia 75-89’. Those fifteen questions of the Summa constitute what is often referred to as the ‘Treatise on Human Nature’, and they cover various aspects of the human soul, its union with the body, its capacities and operations, including the operations of the intellect both in union with and separated from the human body. Nowhere in these questions, nor in the whole Summa of which (as Pasnau notes) they form less than 3 percent, nor indeed in his entire corpus does Aquinas offer an examination or discussion of the issue of intended abortion. Indeed, in all of his voluminous writings there are only three places where aborting a pregnancy is even mentioned, and then only briefly and each in relation to another matter. First, unintended abortion is cited in a quotation from Exodus which forms part of a question on homicide in Summa Theologiae, IIa, IIae, q. 64, a 8. The issue is whether one is guilty of murder through killing someone by chance. Aquinas writes as follows::
Objection 2. Further, it is written (Exodus. 21:22): ‘If . . . one strike a woman with child, and she miscarry indeed . . . if her death ensue thereupon, he shall render life for life’. Yet this may happen without any intention of causing her death. Therefore one is guilty of murder through killing someone by chance.
He then responds:
I answer that: According to the Philosopher (Phys. ii, 6) ‘chance is a cause that acts beside one's intention’. Hence chance happenings, strictly speaking, are neither intended nor voluntary. And since every sin is voluntary, according to Augustine (De Vera Relig. xiv) it follows that chance happenings, as such, are not sins. . . .
Reply to Objection. 2. He that strikes a woman with child does something unlawful: wherefore if there results the death either of the woman or of the animated fetus, he will not be excused from homicide, especially seeing that death is the natural result of such a blow.
Second, again in the Summa (III,a, q. 68, a 11), in a passage paralleling one from the earlier Commentary on the Sentences, Aquinas considers whether in circumstances in which the life of a child in the womb is in danger one may ‘open the mother’ in order to baptize it and thereby equip it for salvation. In both texts Aquinas responds negatively citing the anti-consequentialist ‘Pauline principle’:
Evils are not to be done that goods may come from them, Romans 3 , and therefore a man ought rather to let the infant perish, than that he himself do so by committing the crime of homicide in killing the mother. 
Given the brevity and evidently incidental character of these passages, and the fact that they occur outside the context of Pasnau’s specified text, one may be surprised to find that chapter 4 of his book is mostly concerned with the issue of abortion, and more precisely with the effort to show that Aquinas can be deployed against a ‘pro-life’ stance on the issue. In fact, Pasnau’s discussion is more pointed, being an attack on the official sanctity of life doctrine of the Roman Catholic Church. Since his remarks are fairly extraordinary in the context of a scholarly study it is appropriate to quote them in full (page references are given in parentheses). Pasnau writes:
There is an unfortunate tendency to conflate interest in medieval philosophy especially on the work of Thomas Aquinas, with sympathy for the Roman Catholic Church. Inasmuch as the Church’s intellectual foundations lie in medieval philosophy, above all in Aquinas, sympathy for his work naturally should translate into sympathy for Catholicism. But the conflation is still unfortunate, because in recent years the Church has identified itself with a noxious social agenda - especially on homosexuality, contraception, and abortion - that has sadly come to seem part of the defining character of Catholicism. So it should be gratifying, for students of medieval philosophy, to see how in at least one of these cases Aquinas provides the resources to show something of what is wrong with the Church’s position (p. 105)
And later he remarks:
Aquinas’s view on these matters is not widely known. Those who do know are generally not eager to advertise it, and indeed have often attacked it in scholarly circles (p. 115).
On the same page, Pasnau writes ‘To suppose that the human soul comes into existence at the moment of conception is to endorse, at least implicitly, a highly Cartesian conception of the soul’ (p. 115). In support of this Pasnau cites an article by the Jesuit theologian Joseph Donceel and in the same footnote lists three other articles (by E. H. Kluge, by Thomas Shannon and Alan Wolter, and by William Wallace) as providing similar lines of argument. He continues ‘If these articles were more widely known and appreciated, much of this chapter would be unnecessary’ (fn 19, p. 420).
We shall engage the substantive questions shortly, but given the tone and implications of Pasnau’s remarks so far as concerns both Catholic moral teaching and the integrity and activities of scholars, it is important to address these directly. In summary his suggestions are as follows. First, Catholicism has chosen to advance a ‘noxious social agenda’ at least part of which runs counter to the view of Aquinas, its greatest theologian and a proclaimed Doctor of the Church. Second, those who are aware of the latter fact try to avoid its becoming generally known and in scholarly circles attack Aquinas’s view. Third, there are, however, a few writers who know the facts and are not afraid to announce and argue the case for them, though they have rarely managed to be heard. Fourth, were their contribution properly acknowledged then truth would out and the Catholic Church’s position would be weakened.
Quite contrary to this impression of concealment, however, it is a commonplace of informed, scholarly discussions in this area that Aquinas (along with other ancient and medieval writers) believed in late human ensoulment, often referred to as ‘delayed hominization’, and there is an extensive scholarly and semi-popular literature on it contributed to by parties from different sides of the interpretative, philosophical, theological and moral debates. In another footnote Pasnau mentions two authors (Stephen Heaney and John Finnis) who argue that if Aquinas had known the facts of embryology he would have held that the human soul is present from conception (fn. 11. p. 419). These and those mentioned above are just a few among many others who have contributed to the well-known, and still ongoing debate in Catholic theological and philosophical circles concerning immediate vs. delayed hominization and the relevance to this issue of Aquinas’s views. It is surprising that Pasnau seems unaware of the extent of this literature, not least because the few items he does mention contain references to other works. As it is, his treatment of the matter suggests an overly hasty rush to judgement. Indeed, echoing Pasnau, we are inclined to think that if this literature had been known and appreciated by him then much of the chapter would have been avoided or, at the very least , that it would have been different in tone and substance.
The Evolution of Catholic Teaching
Not only are scholars in the field generally well aware of Aquinas’s views about human ensoulment, but the Catholic Church itself has made reference to these in its public declarations promulgating a contrary position. The latter fact is unsurprising given common knowledge of the medieval view and, as Pasnau notes, the important position of medieval thought in general, and that of Aquinas in particular, in shaping Catholic philosophy and theology. For example, in the historically important and oft-cited 1974 ‘Declaration on Procured Abortion’, the Sacred Congregation for the Doctrine of the Faith writes as follows:
In the course of history, the Fathers of the Church, her Pastors and her Doctors have taught the same doctrine [that human life must be protected and favored from the beginning, just as at the various stages of its development] - the various opinions on the infusion of the spiritual soul did not introduce any doubt about the illicitness of abortion. It is true that in the Middle Ages, when the opinion was generally held that the spiritual soul was not present until after the first few weeks, a distinction was made in the evaluation of the sin and the gravity of penal sanctions. Excellent authors allowed for this first period more lenient case solutions which they rejected for following periods. But it was never denied at that time that procured abortion, even during the first days, was objectively grave fault. This condemnation was in fact unanimous.(emphasis added).
The principle factor in effecting a change in the Church’s teaching about the nature of the (objective) sinfulness of early abortion was the development of knowledge of human embryology. We will return to the specifics of this in the next section, but in general the ancient, medieval and early modern view was that sexual reproduction involved the solidification and formation of menstrual fluid under the influence of the father as mediated by the semen. This led in due course to the production of a body (a recognizable human figure) to which a rational soul was then conjoined. This being the scientific-cum-metaphysical view, a moral distinction could then be drawn between terminating a pregnancy before and after the point of rational ensoulment (hominization). Prior to this the act would be life-destroying but not homicidal; subsequent to it abortion would be the killing of a human being. Such was Aquinas’s view, hence his previously quoted verdict that if one deliberately strikes a pregnant woman, knowing her to be pregnant and knowing of the risk of death, then ‘if there results the death either of the woman or of the animated fetus he will not be excused from homicide’ (emphasis added). Had the blow been delivered prior to animation then the abortion would be sinful but the sin would in effect be that of contraception (understood in the special sense indicated above).
This teaching is reflected in subsequent theological and confessional manuals. In his Summa Theologica Antoninus (1389-1453) Archbishop of Florence, and like Aquinas a Dominican, considers whether homicide can be justified to avoid another evil. His examples include the case of abortion, and he maintains that if a foetus is animate then it is impermissible to kill it so as to save the mother, and impermissible for the mother, even though she may be going to die, to accelerate her death to save the foetus (ST, II, 7, 8). Another Domincan, Silvester Prieras (1456-1523) in his widely referred to Summa summarium draws the distinction between abortion pre- and post-hominization, and offers the direction that in circumstances where it is uncertain which may have been performed the penitent should be required to confess to and be absolved of the greater sin,.but punished according to the lesser one. This is an example of the ‘lenient case solution’ referred to in the 1974 ‘Declaration’. Other later, and also prominent works, such as the Enchiridion sive manuale confessariorum et penitentium of Martinus Azpilcueta (1492-1586) follow the same course. By stages, however, a change of view begins to emerge about the facts on which the pastoral practice was based. In his De formatrice foetus, Thomas Fieinus (1567-1631), a professor of medicine at Louvain, argues that the soul is present from conception. What earlier writers, following Aristotle thought of as a succession of formative principles (souls), each replacing its predecessor, can be viewed as successively emergent functions attributable to a single original principle (brought to life by the effect of intercourse). Fieinus then claims, rather in the style of later critics of delayed hominization, that if there were no rational soul present until the exercise of higher mental functions then one would have to say that rational animation only occurs two to three years after birth. Another influential figure, Paulo Zacchia (1584-1659) argues in Questiones medico-legalis that the soul which organizes the development of the conceptus is internal to it (i.e. not a remote cause such as the father, mediated by an instrumental power in the semen).
Subsequent to this, embryological studies led to a modern understanding of the ovum and the general process of fertilisation. Inevitably, these influenced philosophical and theological thinking about the origin of individual life, and in the course of the nineteenth century the Catholic Church moved towards its current position. That new position, which was increasingly reflected in moral teaching and pastoral direction, was given theoretical support by an anonymous article entitled ‘De animatione foetus’ published in Nouvelle Revue Theologique in 1879. In this the author argues in favour of immediate animation on the basis of biological and philosophical considerations. Citing Fienius and Zacchia in support, the writer develops the line of reasoning (though now put in terms of a fertilized ovum rather than of a preformed embryo) that if the principle of formative development is immanent then animation is immediate. He also tries to show that this is theologically acceptable inasmuch as arguments from scripture and tradition are in themselves inconclusive on the matter. Subsequent Church documents have consolidated this position. Allowing that there may be some indeterminacy in best current accounts of when exactly a new human being begins to exist, the Church nevertheless teaches that this should be deemed to occur at conception. The point is stated clearly in a later declaration of the Sacred Congregation, Donum Vitae ‘The Gift of Life’ - more prosaically described as an ‘Instruction on Respect for Human Life In its Origin and the Dignity of Procreation: Replies to Certain Questions of the Day’:
The Congregation recalls the teachings found in the Declaration on Procured Abortion: "From the time that the ovum is fertilized, a new life is begun which is neither that of the father nor of the mother: it is rather the life of a new human being with his own growth. ...” . . . This teaching remains valid and is further confirmed, if confirmation were needed, by recent findings of human biological science which recognize that in the zygote resulting from fertilisation the biological identity of a new human individual is already constituted.
Thus the fruit of human generation, from the first moment of its existence, that is to say from the moment the zygote has formed, demands the unconditional respect that is morally due to the human being in his bodily and spiritual totality. The human being is to be respected and treated as a person from the moment of conception; and therefore from that same moment his rights as a person must be recognized, among which in the first place is the inviolable right of every innocent human being to life. 
The same passage from the 1974 Declaration, embedded within a quote from this later statement is cited by John Paul II in Evangelium Vitae, ‘The Gospel of Life’ (1995) where he adds
Furthermore, what is at stake is so important that, from the standpoint of moral obligation, the mere probability that a human person is involved would suffice to justify an absolutely clear prohibition of any intervention aimed at killing a human embryo.
Aquinas on Human Ensoulment
So much for the history and evolution of Catholic teaching. What of Aquinas’s view of when human life begins and the question of whether, if it can be restated free of erroneous embryological assumptions, it lends support to Catholic teaching or undermines it?
Aquinas held that in higher animals the efficient cause of generation is the male, while the female is only the material cause: ‘In perfect animals, generated by coitus, the active power [virtus] is in the semen of the male, according to the Philosopher in De Generatione Animalium, but the matter of the fetus is what is provided by the female’. The Aristotelian view was that the menstrual blood provided by the female is nonliving and relatively lacking in organization or differentiation. Thus, the main question Thomas faced was that of how the male causes the generative process, given that this process occurs in the body of the female. It is a general principle of Aristotelian/Thomistic metaphysics, preserved in Descartes’ idea that there must be as much reality in the cause as in the effect,  that a process directed towards the production of an effect must have a fully adequate cause;. one at least equal in perfection to the effect itself. Accordingly, the process of generation must derive from a cause as elevated in the order of nature as is its product. The problem in the case of sexual reproduction arises from the belief that the semen from the male and the menstrual blood from the female are not, by themselves, capable of producing a living animal.
Aquinas adopts Aristotle’s solution: the male is the principal active cause of the generative process, but this cause acts through a medium, the semen, which is therefore an instrumental cause. Within the semen there is an active part which he calls the ‘animal spirit’, a gaseous, airy material entity; and within the animal spirit is a ‘virtus formativa’, a formative power similar to the power in any instrumental cause imparted to it by a principal cause. As in the saw producing a bed there is a power or motion from the carpenter, so in the animal spirit of the semen there is a power or motion from the male, a virtus formativa. This formative power organizes the matter provided by the female in the menstrual blood, first to form a being with vegetative life, then a being with lower sensitive life, and so on, until the organization is produced sufficient for an animal of the same species as the parents. The active part of the semen (the animal spirit) remains until the generative process is completed. This part also obtains heat from the sun, confirming the claim of Aristotle, that ‘man is generated by man and the sun’. Aquinas describes the (imagined) process:
And after the sensitive soul, by the power of the active principle in the semen, has been produced in one of the principal parts of the thing generated, the sensitive soul of the offspring begins to work towards the perfection of its own body, by nourishment and growth. The active power which was in the semen ceases to exist when the semen is dissolved and its spirit vanishes.
He also held that, unlike the souls of brute animals, the human soul is directly created by God. In various places he argues that the rational soul has intellectual powers of conceptual thought that are independent of matter, and hence the operations of these powers are not performed with a bodily organ. Therefore, the rational soul must have its existence independently of matter. But what has existence independent of matter cannot come to be through the coming into existence of a matter-form (or body-soul) composite. Thus, Aquinas held that God immediately creates the human soul and (at the same time) infuses it into the body. That said, the human rational soul is created and infused into the body only when the human parents have, by their generative act, produced a material substance that is disposed to receive and to be informed by a human soul. In one place Aquinas follows Aristotle in saying that the rational soul is infused at 40 days for males, and at 90 days for females.
Why, then, did Aquinas hold that the process of human conception must occur gradually and incrementally? Why did he hold that first vegetative life was produced, then sensitive life, and so on? Why not immediate hominization? The answer lies in his belief that there is a great distance between the beginning point of the generative process, that is, the material out of which the human being is produced (menstrual blood), and the end point, the coming to be of a human being. Traversing this distance requires a gradual process. In one of his fuller treatments of the issue, he writes:
And we must observe a difference between the process of generation in men and animals and in air or water. The generation of air is simple, since therein only two substantial forms appear, one that is displaced and one that is induced, and all this takes place together in one instant, so that the form of water remains during the whole period preceding the induction of the form of air. On the other hand, in the generation of an animal various substantial forms appear: first the semen, then blood and so on until we find the form of an animal or of a man. 
According to Pasnau, Aquinas held that in order for the rational soul to be infused, certain material conditions have to obtain. In particular, while intellectual acts are not themeselves material they depend upon the operations of the senses, and these require a developed brain. Hence the organs upon which the rational soul’s activities rely must be fully developed, in the sense of having their powers not in remote potentiality but ‘in hand’, that is, as immediately exercisable capacities to support intellectual operations. It is indeed true that Aquinas held that organs must be present before human ensoulment. In the Summa Contra Gentiles, for example, he writes:
For, since the soul is united to the body as its form, it is united to the body as its proper act. Now the soul ‘is the act of an organic body’ (Aristotle, II De Anima, 412b, 5-6) Therefore, the soul does not exist in the semen in act [as opposed to in potency or virtually] before the organization of the body.
But this is not to claim that the organs must be developed to the extent of having a directly exercisable capacity to support the operations relevant to them. It is not the same as saying, as Pasnau does, attributing this to Aquinas, that in the generation of human beings the brain must be developed sufficiently for the capacity for conceptual thought to be ‘in hand’ or immediately exercisable. Nowhere does Aquinas assert this stronger requirement; and it is quite unlikely that he held it. As was said, he maintained that the rational soul was present after 40 or 90 days, and it is difficult to think that he really believed that embryos at this early date are actually engaging in conceptual thought, or have the immediately exercisable capacity to do so. Rather, Aquinas’s argument only shows, and he surely only held, that the beginnings, or primordia, of such organs, and in particular, the primordium of the brain, must be present.
As was seen, the key metaphysical principle concerning the question of the time of ensoulment is the following: P) in a material substance the matter must be proportioned to the form, or in a living material substance, to the soul. But this principle could mean two different things:
P. 1) what is necessary for ensoulment is the presence of the actual organs, sufficiently developed to support the operations proper to that species; or
P. 2) what is necessary for ensoulment is the material organization sufficient for the development of those organs, in other words, the epigenetic primordia of the organs that support the operations proper to the species.
Aquinas was unaware that the embryo satisfies P. 2) (has epigenetic primordia of organs) long before it has visibly present organs. He thought that the formative power in the ‘animal spirits’ remained, and acted on the menstrual blood, then on the embryo, then on the fetus, up to the time that the organs were actually present (though not yet operative, or able to operate for some time). Pasnau thinks Aquinas held P. 1), and that this interpretation of the metaphysical principle is true. We have shown that Aquinas did not hold P. 1), though he did apparently require the presence of visible organs. But more importantly, given the embryological facts as we now known them, Aquinas’s principles actually lead to P. 2), and to the conclusion that the rational soul must be present at conception, that is, at fertilisation.
Why might one hold P. 1) instead of P. 2)? Pasnau does not draw precisely this distinction, but he does distinguish a capacity in hand from a capacity to develop a capacity in hand (p. 115). He also maintains that a human being exists only if there is a being with the capacity in hand for conceptual thought; hence that the actual organs must be present and developed sufficiently to support such thought. To suppose otherwise, says Pasnau, ‘is to endorse, at least implicitly, a highly Cartesian conception of the soul’ (p. 115). Indeed, he claims the following:
This moment-of-conception thesis can seem plausible, in other words, only if one endorses the following two claims:
The soul stands apart as an independent substance, housed within the body but not united to the body.
Human beings just are their souls, housed within a body.(pp. 115-116 - emphasis added).
A few lines later he argues that the moment-of-conception thesis has even further implications ‘that would be an embarrassment to the pro-life movement: For if there are no limits on the kind of body that can serve as a subject of the human soul, then there is no reason to suppose that God waits until conception to create such souls’ (p. 116). In other words, Pasnau supposes that one must hold either a) that human ensoulment requires the presence of organs developed sufficiently to support conceptual thought, or b) that any matter at all could be associated with the body, in which case one in effect abandons the position that matter and form truly unite to form one substance, and so one adopts a Platonist or Cartesian view. But this is a straw man. To reject the requirement of a brain sufficiently developed to support an immediately exercisable capacity for conceptual thought (which would place human ensoulment long after birth), is quite different from holding that ‘there are no limits’ on the organization of the matter pre-requisite for the human soul.
Pasnau also argues that ‘At a minimum, it would be pointless for God to infuse the human soul at an earlier point’ (p. 113). This is because, on the Thomistic position, the human mind naturally functions only upon images produced by the senses, imagination and other internal sense powers, and these depend upon a fully developed brain. This argument, however, ignores the fact that the development of the human body is a specifically human function,and therefore requires a human soul as its cause. This is why Aquinas (and Aristotle) held that the semen is only an instrumental cause, while the soul of the male parent is the principal cause. Yet we now know that there is no extrinsic virtus formativa attached to, or residing in the embryo. So, the cause of this development must be the embryo itself. Accordingly, human ensoulment at the time at which the formation of a specifically human body begins is not pointless but necessary.
The claim that human ensoulment does not occur until the brain is sufficiently developed immediately to support conceptual thought cannot be correct, for (at least) two reasons. First, we now know with certainty that the brain is not sufficiently developed to support conceptual thought until some months after birth. So, on this position one would have to say that a six week-old infant is not even a human being, and that is absurd. Pasnau discusses this difficulty, and says that it must also have occurred to Aquinas. He admits that Aquinas did not explicitly discuss this issue; still, he argues that, being aware of the limitations of his knowledge of developmental neurology, Thomas took a conservative stance (p. 119). According to Pasnau: ‘His sense of respect for human life leads him to frame his account as generously as possible, to count within the species even those who are taking the first tiny steps toward full intellectual proficiency’ (p. 120). This approach, says Pasnau, ‘has much to recommend it’; and, moreover, ‘Our own sense of respect for human life should lead us to endorse something like Aquinas’s account’ (p. 120).
But there is absolutely no evidence that Aquinas located human ensoulment where he did out of caution or a ‘sense of respect for human life’. Rather, he simply followed Aristotle on this point, and Aristotle held to this position based on inadequate observation: those were the dates that the embryo seemed—to the naked eye, by necessity—to have organs. But neither Aristotle nor Aquinas could have thought that two or three month old embryonic human beings were engaging in conceptual thought, nor that they had immediately exercisable capacities to do so. Moreover, and what is here more to the point, locating human ensoulment at an early date ‘out of respect for human life’, or out of generosity, does not remove the logical difficulty in this position. As noted, the view Pasnau defends (that the human soul is not present until there is a capacity at hand for conceptual thought) has the absurd implication that six week-old infants are not even human organisms. One cannot evade this difficulty by rejecting the implausible consequence simply ‘out of respect for human life’.
The second, and decisive, difficulty for the position Pasnau defends arises from an issue introduced earlier. Aquinas held that the vital spirit in the semen remains as a distinct agent throughout the entire process of formation until the likeness of the generating parent is induced into the material (or educed from the potentiality of the material). Evidently there must be an adequate, ongoing cause of this formative process. The male parent is no longer present. Therefore, Aquinas held that the father acts by means of the instrumental power in the vital spirit of the semen. To operate, this gaseous or airy material must be present throughout the process (see note 14). Yet we now know that there is nothing in the semen which remains as a distinct agent in the process of the embryo’s development. The result of the fertilisation process is a distinct organism. After fertilisation neither the sperm nor the ovum remains. What exists is a distinct organism which then apparently begins a process of self‑development oriented to the stage of a mature human adult. Nothing of the semen - a fortiori no ‘vital spirit’ - remains attached to the developing embryo. Constituents (chromosomes and cytoplasm) of the sperm and the ovum enter into the make-up of this new organism, but they become its parts or organs. So, if one holds that the embryo is only gradually formed to the point that it becomes apt for the emergence of a sensitive soul and then (the infusion) of a rational soul, one is faced with a complex, organized process which occurs with regularity, but with no apparent cause. If there is no extrinsic agent responsible for the regular, complex development, then the obvious conclusion is that the cause of the process is within, that it is the embryo itself. But in that case the process is not an extrinsic formation, but is an instance of growth or maturation, ie., the active self-development of a whole, though immature organism which is already a member of the species, the mature stage of which it is developing toward.
Pasnau holds that if we specify that men and women have equal parts in generation, then ‘the virtus formativa begins to look very much like DNA’ (p. 103). He compares both to a blueprint for a house: ‘Just as DNA provides a complete blueprint for the body’s development, so the virtus formativa contains every feature of the developing body, but contains it “virtually” or “potentially” rather than actually (SCG II.89.1754)’ (p. 103). But DNA is entirely different from what Aquinas had in mind by the ‘formative power’. The latter is a power in the animal spirits which remain throughout the whole formative process and no longer. That is why it is plausible to view it as an instrumental cause. But the structure of the embryo’s DNA is distinct from that of the mother or of the father, and it remains in this organism throughout its whole life. Hence the DNA is clearly not an instrument of either parent or of both. Rather, the DNA genes are organs of the new organism. Pasnau might have been misled by the analogy of DNA to a blueprint. DNA is similar to a blueprint in one respect, in that it guides the development of the organism. But the obvious disanalogy is that the blueprint does not enter into the structure of the house, whereas the genes which contain the DNA, and whose sequence guides the self-development of the embryo, remain within as parts of the embryo, fetus, infant, adolescent, and so on throughtout life.
Someone might propose that even though nothing of the semen or ovum remains as a distinct agent, nevertheless a virtus formativa persists, somewhat in the way one might conceive an impetus being imparted to a projectile. Thomas himself did not hold the impetus theory as an explanation of the motion of projectiles, though later Thomists proposed it. The general idea of the impetus theory is problematic; yet even if it could be applied in some situations, it could not apply here. The plausibility of the theory depends on thinking of an extrinsic force remaining for a time within a body, moving it in a direction contrary to that to which its intrinsic tendencies incline it. But there is no reason in the case of the embryo to think that the DNA is an extrinsic agent. Unlike the forces operative in a projectile body, the factors responsible for the direction of the embryo’s growth are not transitory, but remain in the developing organism until it dies.
So, can the reasons for Aquinas’s position that human ensoulment occurs after conception (fertilisation) still have force today once they are freed from erroneous embryological assumptions? The reasons which led Aquinas to hold late human ensoulment are basically four, three embryological points and one metaphysical. First, on his Aristotelian view, the male is the sole active cause; second, the material (the menstrual blood) upon which the semen (as instrument of the male) works has only a very low degree of perfection or organization, not even possessing vegetative life; third, as a consequence, the distance between the initial point (menstrual blood) and the end point (a body sufficiently organized to receive a human soul) is quite long. The general metaphysical point is expressed by Aquinas as follows:
Now it belongs to the natural order that a thing is gradually brought from potency to act. And therefore in those things which are generated we find that at first each is imperfect and afterwards is perfected.
We believe that the general metaphysical principle is demonstrably true, and that the application of it in the second sentence is plausibly so. All three of the embryological beliefs, however, are known to be false. Modern embryology shows that the female provides a gamete (the ovum) which is already a highly organized living cell, containing highly complex, specific information, in the genetic structure of the nuclear chromosomes. This information (together with that provided by the genetic structure in the chromosomes of the male sperm) helps guide the development of the new living organism formed by the fusion of the sperm and the ovum. Hence the ovum is actually very close to readiness for rapid embryological development; it only requires fusion with the sperm and the activation that occurs with that fusion. To a certain extent the gradual transition from the simple to the complex that Aquinas sought actually occurs during gametogenesis (of which, of course, he was unaware).Thus, applying Aquinas’s metaphysical principles to the embryological facts uncovered since his time leads to the conclusion that the human being is present from fertilisation on.
Taking Life Seriously
Evidently there is more that could be said about these complex issues, and scope for further debate. The principal purpose of this essay, however, is not to attempt fully to resolve matters, but to take issue with the way in which they are represented in Robert Pasnau’s book: first, with regard to existing philosophical and theological discussions of the issue raised by Aquinas’s account of ensoulment; and second, with regard to the bearing of that account on the substantive question of when human life begins. The morality of abortion turns on two important sets of issues: the first metaphysical, concerning the beginnings of human life and the specific status of the embryo; the second, ethical, having to do with the nature and scope of value and associated moral requirements. Thus far we have addressed Pasnau’s treatment of the former; in ending we wish to say something, in brief, about his remarks concerning the latter.
Pasnau claims that were the abortion debate redirected from the status of the embryo to the issue of the value or ‘precious character’ of human life, then we would be forced to address an urgent issue, namely how to balance the value of human life against other things we value: ‘such as the quality of that life, the lives of future generations, the lives of other animals, the health of the environment’. Observing the increase in world population and the depletion of natural resources he then continues:
[W]e now need to think hard about exactly how to weigh the value of human life. John Paul II [Evangelium Vitae] speaks of ‘the incomparable value of every human person’ and ‘the inestimable value of human life’, but we have reached the point where this sort of rhetoric should be questioned. We can no longer afford not to weigh the value of human life, and in making such estimates, we will be forced into comparisons and trade offs. (p. 125)
As with his earlier remarks about the Catholic Church, this criticism seems gratuitous in a work bearing the title ‘A Philosophical Study of Summa Theologiae Ia 75-89’. In the epilogue, Pasnau again quotes the first of John Paul’s phrases preparatory to seeking once more to deploy Aquinas in opposition to Catholic teaching:
The present Catholic Pope gives voice to an almost universal assumption, across religions and cultures, when he speaks of ‘the incomparable value of every human person’ ...
... this would have been quite foreign to Thomas Aquinas. Far from placing human beings at the center of the moral universe, Aquinas conceives of us as just one small part of creation, excellent in our own way but dwarfed in the grand scheme of things. Far from supposing that God created the universe for our sake, Aquinas believes that we were created for God’s sake, as a manifestation of his goodness ...
[W]e serve a larger, more significant purpose, the manifestation of God’s goodness, and in that larger context we are simply the means to God’s end. Aquinas insists on this clearly and repeatedly: ‘God wills his own goodness as an end, and wills everything else as the means to that end’ [Summa Contra Gentiles I. 86. 718]. (pp. 394 and 395).
It is hard to know what to make of all of this, but impossible to resist the observation that, in the context of Pasnau’s own remarks, the charge of rhetoric is somewhat ironic. It is also unjust. ‘Rhetoric’ nowadays bears two broad meanings: skill in the effective use of speech; and insincere or grandiloquent language. Evidently, he does not mean to criticise John Paul II for exhibiting the first of these. Nor do we suppose that he means to question the Pope’s sincerity. Certainly no-one who has read Evangelium Vitae or Veritatis Splendor or Fides et Ratio - the Pope’s three most philosophical encyclicals - could seriously doubt that John Paul II is genuine in what he says about the value of human life, or suppose that his expressions exaggerate his sincere beliefs. We take it, then, that Pasnau’s non-rhetorical objection is to their content, ie. that he is contesting the claim that human life in general is indeed of ‘inestimable value’ and that each human person really is of ‘incomparable value’. This is what is suggested by his own proposal about the need to weigh the value of human life against other things we value, and to make comparisons and trade-offs.
It is impossible here to provid a defence of ethical absolutism but we can at least show that Pasnau’s representation of the authors he discusses is again contentious, and indicates likely confusions in his own thinking. What John Paul II believes and teaches is represented by the following characteristic passages drawn first from Evangelium Vitae:
Man is called to a fullness of life which far exceeds the dimensions of his earthly existence, because it consists in sharing the very life of God. The loftiness of this supernatural vocation reveals the greatness and the inestimable value of human life even in its temporal phase. ... it is precisely this supernatural calling which highlights the relative character of each individual's earthly life. After all, life on earth is not an ‘ultimate’ but a ‘penultimate’ reality; even so, it remains a sacred reality entrusted to us, to be preserved with a sense of responsibility and brought to perfection in love and in the gift of ourselves to God and to our brothers and sisters. 
and then, from Veritatis Splendor:
[T]he moral life has an essentially ‘teleological’ character, since it consists in the deliberate ordering of human acts to God, the supreme good and ultimate end (telos) of man. ...
The primary and decisive element of moral judgement is the object of the human act, which establishes whether it is capable of being ordered to the good and to the ultimate end, which is God. ...
Reason attests that there are objects of the human act which are by their very nature ‘incapable of being ordered’ to God, because they radically contradict the good of the person made in his image. These are the acts which, in the Church’s moral tradition, have been termed ‘intrinsically evil’ (intrinsece malum): they are such always and per se, in other words on account of their very object, and quite apart from the ulterior intentions of the one acting and the circumstances ... examples of such acts [are] ‘Whatever is hostile to life itself, such as any kind of homicide, genocide, abortion, euthanasia and voluntary suicide; whatever violates the integrity of the human person ... whatever is offensive to human dignity ... all these and the like ... are a negation of the honour due to the Creator’.
In short, for John Paul II to say that human life is of incomparable value is not to ‘place human beings at the center of the moral universe’. On the contrary, it is to say that human beings have this special and inestimable value inasmuch as they are created in the image of God (imago dei) and in order to come to participate supernaturally and eternally in the life of God. That beings should be created with this nature and for this end is indeed a manifestation of God’s goodness. It also explains why even on earth human life is a ‘sacred reality entrusted to us’ and why intentionally to take innocent human life is ‘a negation of the honour due to the Creator’. Far from there being a difference between John Paul and Aquinas on this matter the former frequently cites the latter and uses formulations drawn from the Summa Theologiae, Prima Secundae - such as that ‘all that man is, or has, or can be is ordered to God’ (q. 21).
Moreover, Aquinas certainly did not hold that God created human beings as means to His ends. In saying, as in the text Pasnau quotes, that the purpose of creation is the manifestation or communication of God’s goodness, Aquinas is making a twofold point: 1) it is not that the goodness of creatures causes God to love them (for this would mean that God was subject to the causality of his creatures and hence not perfect); but also 2) they are not means toward attaining some good God lacks (for again this would imply God’s dependence on his creatures and hence his imperfection ). Rather, Aquinas’s position is that, in one and the same act God (necessarily) wills his own goodness and (freely) wills, out of pure generosity, to communicate his goodness to creatures. Creatures are directed to God’s good for their perfection or fulfillment, not for God’s perfection. God necessarily wills (or loves) his own goodness as his end, and creatures as ‘ea quae sunt ad finem’ (those which are toward the end). To call them mediae or means, would imply that creatures are means to an end God does not yet possess, and that he is dependent on creatures for his perfection or fulfillment. Rather, according to Aquinas, creatures are loved out of pure generosity, precisely the opposite of the view Pasnau attributes to him, namely, that they are willed as mere instruments.
In presenting Catholic ‘pro-life’ teaching the Pope also often cites the same passage from Romans 3. 8 as does Aquinas. One such occurrence in the writings of the latter was quoted earlier, namely that ‘Evils are not to be done that goods may come from them, and therefore a man ought rather to let the infant perish than that he himself do so by committing the crime of homicide in killing the mother’(op. cit.). In light of what we have argued above, we believe that had Aquinas addressed the issue of intentional abortion for the sake of saving the mother then his judgement on the matter would have been symmetrical, viz. that evils are not to be done that goods may come from them, and therefore a man ought rather to let the mother perish than that he himself do so by committing the crime of homicide in killing the foetus. This may be a hard teaching, out of keeping with the judgement of the world, presuming a supernatural destiny, and evidently open to question on various grounds. But better to face it as implied by the views of Aquinas than to suggest, quite implausibly, that he may be enlisted as an ally in a campaign against Catholic teaching on abortion.
Franciscan University of Steubenville.
 See Peter Geach ‘Form and Existence’ Proceedings of the Aristotelian Society, 55, 1954-5; ‘Good and Evil’, Analysis, 17, 1956; Mental Acts (London: Routledge & Kegan Paul, 1957); and ‘Aquinas’ in G.E.M. Anscombe and P.T. Geach, Three Philosophers (Oxford: Blackwell, 1961).
 Robert Pasnau, Thomas Aquinas on Human Nature (Cambridge: CUP, 2001).
 Summa Theologiae. Literally translated by Fathers of the English Dominican Province (London: Washbourne & Oates, 1920).
 Commentary on the Sentences, IV, dist. 6, q. 1, ad 4. In the Summa he adds that if the child is still alive after the mother dies then she should be opened in order to baptize it.
 Readers may consult the following (restricted to writings in English): Benedict Ashley, ‘A Critique of the Theory of Delayed Hominisation’, in D. G. McCarthy and A.S. Moraczewski (eds) An Ethical Evaluation of Fetal Experimentation (St Louis: Pope John XXIII Center, 1976); B. Ashley, ‘Delayed Hominisation: A Catholic Theological Perspective’, in R.E. Smith (ed) The Interaction of Catholic Bioethics and Secular Society (Dallas: Proceedings of the XIth Bishops’ Workshop, 1992); B. Ashley and Albert Moraczewski, ‘Is the Biological Subject of Human Rights Present from Conception?’ in P. Cataldo and A. Moraczewski (eds) The Fetal Tissue Issue: Medical and Ethical Aspects (Braintree, MA.: Pope John Center, 1994); B. Ashley, and Albert Moraczewski, ‘Cloning, Aquinas, and the Embryonic Person’, National Catholic Bioethics Quarterly 1 (2001), 189-201; John Connery, Abortion: The Development of the Catholic Perspective (Chicago Loyola UP, 1977); Daniel Dombrowski and Robert Delete, A Brief Liberal, Catholic Defense of Abortion (Urbana: University of Illinois Press, 2000); Joseph Donceel, ‘Abortion: Mediate v. Immediate Animation’, Continuum, 5, 1967; J. Donceel, ‘Immediate Animation and Delayed Hominisation’, Theological Studies, 31, 1970; J. Donceel, ‘A Liberal Catholic View’, in R. Hall (ed) Abotion in a Changing World (New York: Columbia UP, 1970); reprinted in P. B. Jung and T. A. Shannon (eds) Abortion and Catholicism: The American Debate (New York: Crossroad, 1988); Henri de Dorlodot, ‘A Vindication of the Mediate Animation Theory’, in E.C. Messenger (ed) Theology and Evolution (London: Sands and Co. 1949); John Finnis, Aquinas: Moral. Political and Legal Theory (Oxford: OUP, 1998); Anthony Fisher, ‘“When Did I Begin?” Revisited’, Linacre Quarterly, 58, 1991; Norman Ford, When Did I Begin? (Cambridge: CUP, 1988); Rudolph Gerber, ‘When is the Human Soul Infused?’, Laval Theologique et Philosophie, 22, 1966; Germain Grisez, Abortion: the Myths, the Realities and the Arguments (New York: Corpus, 1966); G. Grisez, ‘When do People Begin?’, Proceedings of the American Catholic Philosophical Quarterly, 63, 1990; Stephen Heaney, ‘Aquinas and the Presence of the Human Rational Soul’, The Thomist, 56, 1992 reprinted in S. Heaney ed., Abortion: A New Generation of Catholic Responses (Braintree, MA.: Pope John Center, 1992); Mark Johnson, ‘Reflections on Some Recent Catholic Claims for Delayed Hominization’, Theological Studies 56 (1995), E. H. Kluge, ‘St Thomas, Abortion and Euthanasia: Another Look’, Philosophy Research Archives, 7, 1981; Patrick Lee, Abortion and Unborn human Life (Washington, DC.: Catholic University of America Press, 1996); Richard McCormick, ‘Who or What is the Pre-Embryo?’, Kennedy Institute of Ethics Journal, 1991; William E. May, ‘‘The Moral Status of the Embryo’, Linacre Quarterly, 59, 1992; John Noonan, ‘An Almost Absolute Value in History’, in J. Noonan ed. The Morality of Abortion (Cambridge, MA.: Harvard U P, 1970); Gabriel Pastrana, ‘Personhood and the Beginning of Human Life’, Thomist 41 (1977); Jean Porter, ‘Individuality, Personal Identity, and the Moral Status of the Preembryo: A Response to Mark Johnson’, Theological Studies 56 (1995); Thomas Shannon and Alan Wolter, ‘Reflections on the Status of the Pre-Embryo’ Theological Studies, 51, 1990; C. Tauer, ‘The Tradition of Probabilism and the Moral Status of the Early Embryo’, Theological Studies, 45, 1984, reprinted in P. B. Jung and T. A. Shannon (eds) Abortion and Catholicism: The American Debate (New York: Crossroad, 1988); Francis Wade, ‘Potentiality in the Abortion Discussions’, Review of Metaphysics, 29, 1975; William Wallace, ‘Nature and Human Nature as the Norm in Medical Ethics’, in E. Pellegrino (ed) Catholic Perspectives on Medical Morals (Dordrecht: Reidel, 1989); William Wallace OP, ‘Aquinas’s Legacy on Individuation, Cogitation and Hominisation’, in D. Gallagher (ed) Thomas Aquinas and His Legacy (Washington, DC.: Catholic University of America Press, 1994); Thomas Wassmer, ‘Questions about Questions’, Commonweal, 86, 1967; Beverly Whelton, ‘Human Nature, Substantial Change, and Modern Science: Rethinking When a New Human Life Begins’, in M. Baur (ed) Texts and Their Interpretation, Proceedings of the American Catholic Philosophical Association, 72, 1998; and Gordon Wilson, ‘Thomas Aquinas and Henry of Ghent on the Succession of Substantial Forms and the Origin of Human Life’, in L. Schrenk (ed) The Ethics of Having Children, Proceedings of the American Catholic Philosophical Association, 63, 1990. Anyone interested in how widely Aquinas’s position on ensoulment is referred to in semi-popular presentations by parties on both sides of the abortion debate may conduct an internet search using some such expression as ‘Aquinas on Abortion’.
 Congregation for the Doctrine of the Faith, Declaration on Procured Abortion (Vatican: Holy See, 1974) II, 7.
 Since Aquinas regards contraceptive and homosexual acts as objectively gravely sinful, it may be supposed that these elements of Catholic teaching are unlikely to be vulnerable to challenge by reference to Aquinas. It should come as no surprise, however, that some writers have tried to do just this. Interestingly, Pasnau does not make reference to these attempts. One difference in the cases may lie in the fact that in these instances Aquinas’s announced moral view is not tied to obviously false empirical assumptions.
 For accounts of these and related views see Grisez, Abortion: the Myths, the Realities and the Arguments; Connery, Abortion: The Development of the Catholic Perspective, and Dombrowski and Delete, A Brief Liberal Catholic Defese of Abortion, op. cit.
 Congregation for the Doctrine of the Faith, Donum Vitae (Vatican: Holy See, 1987) I, 1.
 John Paul II, Evangelium Vitae (Vatican: Holy See, 1995) III, 60.
 Summa Theologiae, Ia, q. 118, a. 1, ad 4.
 Descartes Meditations, III, 14
 ‘Consequently there is no need for this active force to have an actual organ; but it is based on the spirit in the semen which is frothy, as is attested by its whiteness’, Summa Theologiae, Ia, q. 118, a. 1, ad 3.
 ‘In which spirit [contained in the semen], moreover, there is a certain heat derived from the power of the heavenly bodies, by virtue of which the inferior bodies also act towards the production of the species as stated above. And since in this spirit the power of the soul is concurrent with the power of a heavenly body, it has been said that “man and the sun generate man”’. Summa Theologiae, Ia, q. 118, a. 1, ad 3.
 Summa Theologiae, Ia, q. 118, a. 1, ad 4. Cf. Summa Contra Gentiles transl. James F. Anderson (London: University of Notre Dame Press, 1975), Bk. II, Ch. 89, 9; Disputed Questions on the Power of God, transl. English Dominican Fathers (Westminster, MD: Newman Press, 1952), q. 3, a. 11, ad 8.
 Aristotle also held that the rational soul had to come ‘from outside’. See On the Generation of Animals, Bk. II, ch. 3, 736b25-30. For a contemporary argument see J. Haldane, ‘Old Teleology’ and ‘The Prime Thinker’ in J.J.C. Smart and J. J. Haldane, Atheism and Theism, 2nd ed. (Oxford: Blackwell, 2003) 88-109, and 227-32
 The ultimate disposition is provided by the soul itself, but the matter it acts on must be disposed for this ultimate disposing act.
 Commentary on the Book of Sentences, Bk. III, dist. 3, q. 5, a. 2, Responsio.
 On the Power of God, q. 3, a. 9, ad 9. Cf. Summa Contra Gentiles, II, Ch. 89, 11; Summa Theologiae, Ia, q. 119, a. 2.
 Pasnau writes that Aquinas held that the human soul ‘is infused at that point when the fetus is sufficiently developed in its brain and sensory systems, to support the soul’s intellectual operations’. (p. 111)
 Summa Contra Gentiles, II, ch. 89.
 One might argue that the organs must be fully present, though not yet capable of immediately operating. But this is not really an option, for to be fully present just is to be capable of immediately operating, though environmental conditions may not be suited to the operation. For the standard of completeness of an organ is its function. Hence the only intelligible proposals are, either 1) the organs must be developed sufficiently to operate immediately or 2) that the epigenetic primordia of the organs must be present.
 Citing recent works on the development of the neurosystem of the fetus and newborn, Pasnau admits that there is evidence that the brain is in fact not sufficiently developed to support conceptual thought until at least three to six months after birth (p. 119). Pasnau cites Colwyn Trevarthen, ‘Brain Development’, in R. Gregory, (ed) Oxford Companion to the Mind (Oxford: OUP, 1987), 101-110, and Stuart Derbyshire, ‘Locating the Beginnings of Pain’, Bioethics 13 (1999), 1-31.
 See B. Ashley, ‘A Critique of the Theory of Delayed Hominization’, op. cit., 121-5.
 Summa Theologiae, Ia q. 119, a. 2.
 It has often been argued that monozygotic twinning shows that the embryo in the first several days of its gestation is not a human individual. The argument is that it is not yet determined which one, perhaps of two, the zygote will be identical with; and so what exists up to about day 13, after which twinning does not seem possible, is a group of cells from which one or more human beings will develop, but is not yet an individual organism, hence not yet a human being. Pasnau adopts this position in a footnote: ‘There is a further question of whether the fertilized egg has the requisite unity, from the start, to be a substance. It seems clear that it does not. At its very early four-cell stage, for instance, each of the four cells, if split from the others, could independently develop into a normal embryo. . . . At the eight-cell stage, however, specialization already sets in, and not just any cell could split off and develop into an independent embryo. Yet even here, fission and fusion remain possible and such processes would seem to violate Aquinas’s criterion for substancehood’. (p. 422, n. 25). It is true that if a cell or group of cells are detached from the whole at this early stage then what is detached often becomes a distinct organism and has the potential to develop to maturity as distinct from the embryo from which it was detached (this is the meaning of “totipotent”). But this fails to show that before detachment the cells within the embryo, constituted only an incidental mass. Just as the fact that dividing a flatworm results in two whole flatworms does not show that prior to that division the flatworm was not a unitary individual, just so with the human embryo. Parts of a flatworm have the potential to become a whole flatworm when isolated from the present whole of which they are part. Likewise, at the early stages of development of the human embryo the degree of specialization by the cells has not progressed very far, therefore the cells or groups of cells can become whole organisms if they are divided and have an appropriate environment after the division. But that does not indicate that prior to such an extrinsic division the embryo is a mere mass of cells rather than a single, complex, actively developing human organism. The clearest evidence that the embryo in the first two weeks is not a mere mass of cells but is a unitary organism is this: if the individual cells within the embryo before twinning were each independent of the others, there would be no reason why each would not regularly develop on its own. Instead, these allegedly independent, non-communicating cells regularly function together to develop into a single, more mature member of the human species.This fact shows that interaction is taking place between the cells from the very beginning (even within the zona pellucida, before implantation), restraining them from individually developing as whole organisms and directing each of them to function as a relevant part of a single, whole organism continuous with the zygote. Thus, prior to an extrinsic division of the cells of the embryo, these cells together do constitute a single organism. So, the fact of twinning does not show that the embryo is a mere incidental mass of cells. Rather the evidence clearly indicates that the human embryo, from the zygote stage forward, is a unitary, human organism.
 John Paul II, Evangelium Vitae (Vatican: Holy See, 1995) Introduction, 2.
 John Paul II, Veritatis Splendor (Vatican: Holy See, 1993) Ch. Two, IV, 73, 79, 80.
 See Summa Theologiae, Ia, q. 19, aa. 2-5.