Though Copernicus has been dead for over 450 years, we continue to place ourselves at the center of the moral universe. According to the great moral theories of our time, it is we human beings who are the measure of good and evil. For Mill, our pleasure is “the ultimate end, with reference to and for the sake of which all other things are desirable” (Utilitarianism, p. 262). For Kant, all of morality can be derived from the principle that human beings should be treated “never merely as a means, but always at the same time as an end” (Groundwork, p. 38). 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” (John Paul II, 1995; see §4.3 at end).

None of these accounts limits the scope of morality to human beings. Mill allows that the pleasure of other animals might carry some moral weight. Kant thinks that his categorical imperative applies to all rational beings that might exist. The Pope takes the value of human life to derive from God. But these three very different thinkers all agree in the absolute value they place on humanity. Though they clash dramatically in their evaluations of what the good for human beings consists in, they agree in giving us an ultimate standing in the moral calculus.

All this would have been quite foreign to Thomas Aquinas.

Aquinas believes that the human soul is immortal. His principal argument for this conclusion, in 75.6, rests on the soul's unique status as a form that is also a substance (§12.1). But to establish that the soul continues to exist apart from its body, he needs to show that the soul continues to function apart from its body. Q89 is devoted to explaining how the intellect continues to function in this way (§12.2). But the fact that a separated intellect can continue to function does not show that the soul survives death, and so Aquinas has to explain how such an intellect can still be considered a human soul (§12.3). He then faces the further problem of explaining how a separated soul can serve to sustain the existence of the human being. In fact, Aquinas believes that a separated soul is not a human being; the human being ceases to exist at death, and will come back into existence only with the resurrection of the body. Reflection on this doctrine sheds light on how, for Aquinas, material substances preserve their identity over time (§12.4).

Incorruptibilis

All human beings surely hope for life after death. What we hope for, quite simply, is that we ourselves will continue to exist, in some reasonably pleasant venue. Although the hope can be simply stated, it is a philosophically complex matter to determine what would have to be the case to ensure such personal survival. This chapter explores Aquinas's account of these issues.

Aquinas offers a sophisticated and rewarding account of how we know our own minds. Although he denies that the mind has direct access to its own nature (§11.1), he believes that through indirect methods the mind can understand itself. The reflective method he proposes is interesting because it opposes the introspective model, but at the same time accounts for the special access we so clearly do have to our own minds (§11.2). In advancing a theory of this form, Aquinas defends an approach that falls in between the two extremes of direct introspection and behaviorism. This moderate approach accounts for why it is so hard for us to understand the mind (§11.3). Moreover, the approach can be extended into a general explanation of how we understand other minds (§11.4).

The inscrutable self

Aquinas devotes QQ84–86 to our knowledge of the external, material world; in Q87 he turns to our knowledge of our own minds. The former series of questions had begun by asking whether the mind does in fact have knowledge of external, material things. Aquinas concluded that we do have such knowledge, in a certain way: “Therefore, it must be said that the soul, through intellect, cognizes bodies by means of a cognition that is immaterial, universal, and necessary” (84.1c; §10.1). In Q87 there is no parallel article devoted to the fundamental question of whether the mind has self-knowledge. Aquinas simply takes such knowledge for granted: his idea, presumably, is that the previous twelve questions (QQ75–86) make the best case he knows how to make for the mind's ability to understand itself.

Although there is nothing problematic in principle about attributing a will to human beings (§8.1), Aquinas makes considerable trouble for himself by insisting that the will is a strictly rational appetite – that in some sense it never goes against reason (§8.2). This leaves Aquinas with the need to explain the obvious and familiar fact that people are sometimes overcome by temptation, even when they apparently know better (§8.3). His account of weakness of will derives much of its plausibility from his insistence that human reasoning is not confined solely to the mind, but relies crucially on the internal senses of the brain, especially imagination and the cogitative power (§8.4). The passions, which influence our choices through these internal senses, can be controlled with difficulty and must be controlled, if we are to lead a virtuous life. But this does not mean that a virtuous person can (or should) live without emotion (§8.5).

Is the will a myth?

The previous chapter took for granted that Aquinas is entitled to the notion of a will, a separate faculty of the soul through which we form desires for the things that reason perceives to be good. It is common enough in modern philosophy to talk about free will while holding onto grave doubts about the propriety of talking about will. The locus classicus for such doubts is Gilbert Ryle (1949), who rails against the trinitarian dogma of a Mind or Soul composed of three parts, Thought, Feeling, and Will.

The object of intellect is the quiddity or common nature of physical objects. Despite the metaphysical difficulties, Aquinas must locate these quiddities within particular material substances (§10.1). The intellect grasps the essential features of substances through information acquired from the senses, but Aquinas's empiricism is tempered by his appeal to a form of divine illumination (§10.2). Although the possible intellect starts out as a blank slate, the agent intellect reveals the natures of things through abstraction. On inspection, the role of agent intellect turns out to be obscure, and abstraction rather mysterious (§10.3). But a clear understanding of the different forms of universality shows how Aquinas's account is genuinely explanatory (§10.4). The main part of understanding takes place not in the agent intellect but in the possible intellect, through the long, hard process of reasoning (§10.5).

Quiddities

Aquinas's methodology – familiar by now – is to work from objects to actions, and from actions to capacities:

It is high time, then, to say something about the object of intellect. Surprisingly, the Treatise does not explicitly address this issue.

Q77 is perhaps the most difficult question in the Treatise, at least at the entry level. The issues it raises about “the capacities of the soul in general” look abstruse and uninviting, concerning matters such as the nature of a capacity (§5.1). and the relationship between such capacities and the soul's essence (§5.2). But although Q77 does not raise the obviously big issues found in other parts of the Treatise, it would be a mistake to dismiss this question as a mere metaphysical balancing of the books. The ideas developed here are fundamental to Aquinas's thinking about human nature, not just in 1a but in the later treatments of virtue that dominate the Summa's second part. The distinction between the soul's capacities and its essence would later prove to be one of the most controversial aspects of the theory (§5.3). Without this distinction, the Treatise would face a crippling circularity (§5.4). Moreover, Aquinas's focus on the soul's capacities is a consequence of an important methodological principle: that the soul's essence is not directly knowable (§5.5).

What is a capacity?

In the De anima, Aristotle offers the following remark in the course of making a transition from his general treatment of the soul to his treatment of the individual powers of the soul:

The longest article of the Treatise, 76.1, is devoted to accounting for the unity of soul and body. Much of the article works to establish the unworkability of accounts on which soul and body are two things that must somehow be tied together (§3.1). Aquinas's own solution is a form of Aristotelian hylomorphism that is meant to guarantee soul-body unity. To understand this special sort of unity, one has to understand Aquinas's general theory of substantial forms, which imposes strict constraints on what counts as a genuine substance (§3.2). The soul is a substantial form, on this account, because all the parts of the body have life and existence if and only if they are actualized by the soul (§3.3). But this unified account of soul and body by no means pushes Aquinas toward reductive materialism; instead, a proper understanding of form shows precisely why reductive materialism is false (§3.4).

The failure of nonreductive theories

After considering the soul in its own right (Q75), Aquinas turns to the relationship of soul and body. More specifically, he turns to “the soul's union with the body” (76pr). To say that soul and body are united (unitur) is simply to say that they make one (unum) thing. The first article of Q76 focuses on how this union is accomplished, taking for granted that soul and body do in fact make one thing.

Everything that has a form has an appetite, but appetites come in different kinds. Natural appetites are fixed by nature, and determine their possessor to the pursuit of a certain end. An end is pursued only if it exerts some causal influence on the pursuer, the influence of final causality, and this requires some sort of cognitive grasp of the end. Hence natural appetites imply an intelligent creator (§7.1). Voluntary agents can determine their own appetites, by gathering information about the world around them. Lower animals meet this standard, and in a sense they can even be considered free agents (§7.2). But genuine voluntariness, and genuine freedom of choice, requires rationality. Indeed, human beings have free choice precisely because we have the capacity to deliberate rationally about the judgments we make (§7.3). Aquinas's account of freedom implies that one can still choose freely, even if one's choice is causally determined. We are in control of our choices so long as our judgments are subject to prior judgments, and our choices subject to prior choices (§7.4).

Natural appetite

Of the human soul's various capacities, only two – intellect and will – distinguish us as human. Aquinas quotes with approval John Damascene's remark that we are made in the image of God in virtue of being “intellectual, free in our decisions, and capable on our own” (1a2ae pr). The latter two of these characteristics depend on the capacity of will. Although much of the Treatise is devoted to intellect, and although Aquinas returns later in ST (1a2ae 6–17) to a discussion of the will, he could hardly have surveyed human nature without at least touching on our so-called rational appetite.

The human soul is subsistent, which is to say that it is in some way an independent substance. This does not mean that we are our souls, because human beings have an essential bodily component (§2.1). Our soul's subsistence is a matter of its having a function – thought – that it performs on its own, independently of the body. More hangs on this argument than on any other passage in the Treatise, but it unfortunately seems to be one of the weaker arguments of the Treatise (§2.2). Unlike the rational soul, the souls of other animals are not subsistent, because sensation requires the body. Indeed, for Aquinas, sensation is a wholly corporeal activity. Aquinas is therefore a materialist regarding sensation, whereas he rejects materialism in the case of the mind (§2.3). Yet Aquinas is not a dualist in any meaningful sense, and his account of the soul-body relationship is an entirely consistent application of his broader metaphysical principles (§2.4).

The essential bodily component

Aquinas begins the Treatise with a striking characterization of his subject matter. Human beings, he says, are “composed of a spiritual and corporeal substance” (75pr). It is hard to imagine, at first glance, a clearer statement of the dualist doctrine: the human mind is one thing, the human body another, each its own independent substance. This reading seems to find confirmation in 75.2, where Aquinas argues that the human soul subsists on its own.

The nutritive and sensory capacities in a human fetus are a natural result of procreation (§4.1). But unlike other animals, human beings are only partly a product of natural biological processes. The rational soul is infused by God once the fetus has developed sufficiently to receive such a soul (§4.2). Aquinas believes that this rational soul, rather than completing the developing fetus, corrupts and replaces that fetus with a different, now human, substance. These views have interesting implications for the modern abortion debate, in that they show how traditional theological conceptions of God and soul actually give us reason to deny that early-term fetuses are human beings (§4.3). All of this is underwritten by Aquinas's claim that a human being has only one substantial form, combining all of the capacities of the nutritive, sensory, and rational parts (§4.4).

Conception

The main challenge in interpreting Aquinas's account of the soul-body relationship has been to resist the appearance of an entirely ad hoc theory. The human soul's union with the body should follow as a natural consequence of more general principles about the nature of life, matter, and form. But, of course, human beings are exceptional, in that we combine the immateriality of an angel with the materiality of brute animals. We are, to use Nietzsche's phrase, a hybrid of plant and ghost (Zarathustra prologue, ch. 3).

The first four chapters have each hinted at a deeper metaphysics that I believe lies implicit in Aquinas's thinking about human nature. On this deeper picture, all there is is actuality of various kinds. Material beings are not composites of actuality plus some kind of elusive stuff known as matter, they are instead just composites of certain sorts of actuality. Reality is actuality all the way down, and substances are bundles of actuality, unified by organization around a substantial form.

All actuality derives its existence from God. “The first completely perfect actuality, which has in itself the whole fullness of perfection, causes actual existence in all things” (QDSCic). The physical world is one manifestation of actuality, but there are other manifestations, nonphysical ones. Different forms of actuality exist at various removes from God. Those that are closest are the spiritual substances, the angels. The more things become material, the less actuality they possess. Prime matter is “the most incomplete of all beings” (ibid.); this is the external limit to being, since prime matter has no existence and no actuality at all. To ascribe matter to some being is not to say that it has actuality plus some additional stuff, its material stuff. Instead, to be material is to be actual in a certain limited, inferior way. Matter is no more than a particular manifestation of actuality: complex actuality in motion, subject to alteration, generation, and corruption.

Prime matter is therefore just a logical abstraction, asymptotic to the curve of being.

Introduction

William of Ockham was born around 1287, probably in the village of Ockham near London. After entering the Franciscan order he studied at Oxford and completed the requirements for the master's degree in theology He never became a regent master, however, probably because of the need to defend his teachings on grace, free will, and other topics from charges of heresy brought against them at the papal court in Avignon in the mid-1320s. While Ockham was in Avignon, he concluded that Pope John XXII was himself a heretic on points of doctrine held by most Franciscans at the time to be essential to Christian belief. Ockham fled the curia in 1328 with the Minister General of his order and a few confreres, taking refuge with Ludwig of Bavaria, who was at odds with the papacy over his title as Roman emperor. Ockham then wrote against the teachings of John XXII and John's successor Benedict XII, composed a massive treatise in dialogue form on heresy (with special reference to papal heresy), and discussed at some length the basis and functions of secular and religious governmental institutions. He is usually thought to have died in Munich in 1347 during an outbreak of plague.

Ockham was formally excommunicated for leaving Avignon without permission and was implicitly condemned as a heretic for his opposition to John XXII's bulls against the dominant Franciscan conception of the poverty of Christ, the apostles, and the Franciscans themselves.