Aquinas On Being, Goodness, And Divine Simplicity

Abstract

Aquinas's virtue-based ethics is grounded in his metaphysics, and in particular in one part of his doctrine of the transcendentals, namely, the relation of being and goodness. This metaphysics supplies for his normative ethics the sort of metaethical foundation that some contemporary virtue-centered ethics have been criticized for lacking, and it grounds an ethical naturalism of considerable philosophical sophistication. In addition, this grounding has a theological implication even more fundamental than its applications to ethics. That is because Aquinas takes God to be essentially and uniquely being itself. Consequently, on Aquinas's view, God is also essentially goodness itself. Aquinas's metaphysical grounding for his ethics is thus meant to be understood in connection with his more fundamental views regarding God's nature, and in particular his views of God's simplicity. This metaphysical grounding confers significant philosophical and theological advantages on his ethics.

Introduction

Aquinas's rich virtue-based ethics is grounded in his metaphysics, and in particular in one part of his doctrine of the transcendentals, namely, the relation of being and goodness.¹ This metaphysics supplies for his normative ethics the sort of metaethical foundation that some contemporary virtue-centered ethics have been criticized for lacking,² and it grounds an ethical naturalism of considerable philosophical sophistication.³ The central metaethical thesis of Aquinas's metaphysics of the transcendentals being and goodness is that

‘being’ and ‘goodness’ are the same in reference (idem secundum rem), but differ only in sense (differunt secundum rationem tantum).⁴

This thesis has a theological interpretation more fundamental than any of its other applications to ethics, however. That is because Aquinas takes God to be essentially and uniquely being itself (ipsum esse). Consequently, on Aquinas's central metaethical thesis God is also essentially goodness itself.⁵ Aquinas says,

‘for God alone, [his] essence is his being…And so he alone is good through his essence’.⁶

And elsewhere he says,

‘God is identical with (idem quod) his essence or nature….God is his own deity, his own life, and whatever else is predicated in this way of God’.⁷

Aquinas's central metaethical thesis about the transcendentals being and goodness is thus meant to be understood in connection with Aquinas's more fundamental views regarding God's nature, and in particular his views of God's simplicity. The thesis confers significant philosophical and theological advantages on his ethics. For example, it entails a relationship between God and morality that avoids the embarrassments of both theological subjectivism and theological objectivism and provides a basis for an account of religious morality different in important ways from the more commonly known divine command morality discussed by contemporary philosophers of religion. But, of course, this is the case only if the central metaethical thesis is combined with the doctrine of divine simplicity, as Aquinas means it to be. The doctrine of simplicity is not viewed with much favor in contemporary philosophy of religion, however, primarily because it seems thoroughly counter-intuitive or even incoherent.⁸ In attributing a radical unity to God, and to God alone, it rules out the possibility of there being in God any of the real distinctions on the basis of which we make sense of other real things. Consequently, it has seemed to many philosophers and theologians to give rise to paradoxical or flatly inconsistent conclusions. In this paper, I want to present the theological context of Aquinas's central metaethical thesis and his views of the transcendentals being and goodness by exploring his interpretation of the doctrine of simplicity.

The Claims of the Doctrine of Divine Simplicity

The doctrine of simplicity, as Aquinas understands it, can be sorted out into several specific theses, the most important of which can be summarized in three claims.

The first distinguishes God from material objects:

1. (1) It is impossible that God have any spatial or temporal parts that could be distinguished from one another as here rather than there or as now rather than then, and so God cannot be a physical entity.

Aquinas denies that there is any matter in God or that God has any dimensions,⁹ and so he rules out spatial parts in God. In addition, Aquinas derives divine eternality, which includes God's being outside of time, from divine immutability,¹⁰ which he derives in turn from divine simplicity.¹¹ On Aquinas's view, then, the doctrine of simplicity also has the implication that God has no temporal parts.

Next, the standard distinction between an entity's essential and intrinsic accidental properties cannot apply to God:

1. (2) It is impossible that God have any accidental properties.

Aquinas says,

‘there can be no accident in God. … a subject is related to an accident as potentiality to actuality, for with regard to an accident a subject is in actuality in a certain respect. But being in potentiality is entirely removed from God…’.¹²

Thirdly, the doctrine of simplicity as Aquinas understands it rules out the possibility of components of any kind in the essence that is the divine nature. Even when it has been recognized that all God's intrinsic properties must be essential to him, it must be acknowledged as well that

1. (3) whatever can be intrinsically attributed to God must in reality just be the unity that is his essence.

On Aquinas's view, then, God is his own essence or nature.¹³ For Aquinas, it is impossible that there be any real distinction between one essential property and another in God or between God and his nature. Furthermore, for all things other than God, there is a difference between what they are and that they are, between their essence and their existence; but on the doctrine of simplicity the essence which is God is not different from God's existence. Therefore, unlike all other entities, God is his own being.

In these claims, the counter-intuitive character of absolute simplicity emerges clearly.¹⁴ From those claims it seems to follow, for instance, that God's knowledge is identical with God's power and also with anything that can be considered an intrinsic property of his, such as one of God's actions – his talking to Cain, for instance. And it is not only the drawing of distinctions among God's attributes or actions that is apparently misleading. God's talking to Cain is evidently not really an action of God's; it is his essence, and God himself is that essence. These unreasonable apparent implications of the doctrine of simplicity lead to further embarrassments for the doctrine. If God's talking to Cain is essential to God, it is apparently necessary and thus not something God could refrain from doing. So if in accordance with the doctrine of simplicity each action of God's is in all its detail identical with the divine essence, the doctrine apparently entails that God could not do anything other or otherwise than he actually does. But in what sense then can God be said to have free will? The doctrine of simplicity as Aquinas understands it thus seems at least in tension with other doctrines Aquinas also espouses, such as the doctrine that God's creating was an act of free will on his part.

Resolving Some of the Difficulties

Many, but not all, of these counter-intuitive conclusions can be dispelled by clarifying the claims of the doctrine of simplicity on Aquinas's interpretation of it.

According to Aquinas, in virtue of being absolutely perfect God has no unactualized potentialities but is entirely actual, or in act. Nevertheless, the atemporal pure actuality that is God can have various manifestations and effects in time.¹⁵ It is in that way that there is a mistake in thinking of God's talking to Cain as one of the things God does in the strict sense in which a temporal agent's action is an intrinsic property of the agent's. Aquinas gives this as a standard characterization of the single divine action: ‘God wills himself and other things in one act of will’.¹⁶ As Aquinas understands it, God's willing himself and other things consists in God's willing at once, in one action, both goodness and the manifestation of goodness.¹⁷ But there is no special difficulty in understanding goodness to be manifested differently on different occasions or to different persons. On Aquinas's view, the multiplicity of the objects of God's will is no more in tension with his simplicity than the multitude of the objects of his knowledge is.¹⁸

The absence of real distinctions among divine attributes such as omnipotence and omniscience is to be explained along roughly analogous lines. According to the doctrine of simplicity, what human beings call God's omnipotence or God's omniscience is the single eternal entity considered under descriptions they find variously illuminating, or recognized by them under different kinds of effects or manifestations of it. What the doctrine of simplicity requires one to understand about all the designations for the divine attributes is that they are all identical in reference but different in sense, referring in various ways to the one actual entity which is God himself or designating various manifestations of it. So Aquinas says,

‘the names said of God are not synonymous. This would be easy to see if we were to say that names of this sort are used to separate [attributes from God] or to designate a relation of cause with respect to creatures, for in this way there would be various meanings (rationes) of these names in accordance with various negations or various effects denoted. But since it was said that names of this sort signify the divine substance, although imperfectly, it is also clearly evident that … they have different meanings’.¹⁹

What Aquinas says about being and goodness in his central metaethical thesis thus applies, in his view, to all the terms applied to God: they are the same in reference, but different in sense.

But the difficulties for absolute simplicity canvassed so far are the easy ones. The hardest one to resolve is the apparent incompatibility of God's simplicity and God's free choice. For all I have said so far, the doctrine of simplicity still seems to entail that the only things God can do are the things he does in fact do.²⁰

The Apparent Incompatibility of Simplicity and Free Choice

Since no one whose will is bound to just one set of acts of will makes real choices among alternative acts, it looks as if accepting God's absolute simplicity as a datum leads to the conclusion that God has no alternative to doing what he does. If we begin from the other direction, by taking it for granted that God does make choices among alternatives, it seems that God cannot be absolutely simple. For the doctrine of divine free choice can be construed as the claim that some of God's properties are properties he chooses to have – such as his being the person who talks to Cain at t1. But it makes no sense to suppose that God freely chooses all his properties, so that it is up to him, for example, whether or not the principle of non-contradiction applies to him, or whether he is omnipotent, good, eternal, or simple. Considerations of this sort evidently require us to draw a distinction between two groups of characteristics attributed to God: those that are freely chosen and those regarding which God has no choice.

And this distinction, it seems, cannot be explained as only a reflection of diversity in the temporal effects brought about by the single eternal activity which is God, or as no more than different manifestations of a single active goodness. Instead, this distinction appears to express a radical diversity within divine agency itself, in that some truths about God – such as that he exists – are not subject to his control, while others – presumably such as that he talks to Cain at t1 – are consequences of his free choice.²¹

Nor can this distinction be explained away as an instance of referring to one and the same thing under different descriptions in ways suited to human minds, which can acquire only fragmentary conceptions of the absolute unity that is God. Recourse to the human point of view appears to be unavailable as a basis for explaining the apparent distinction between necessary and freely chosen divine acts of will. Moves in that direction would either present the necessary acts as really indeterminate or deny free choice to God, by suggesting that the appearance of free choice in God is really only a consequence of certain extrinsic accidental properties of his or by presenting the apparently freely chosen acts as not really choice on God's part.²²

Nonetheless, it is abundantly clear that Aquinas takes God to be possessed of choice or liberum arbitrium; ²³ he argues for it vigorously in a variety of places. It is also clear that for Aquinas liberum arbitrium is the power for choosing among alternative possibilities. In addition to the standardly cited passage in ST I q.19 a.10, for example, Aquinas says in QDV q.24 a.3,

‘there remains to God a free judgment for willing either this or that, as there is also in us, and for this reason we must say that free choice is found in God’.

In particular, Aquinas holds that God was free to create or not to create, that God's creating was not brought about in God by any necessity of nature.²⁴

Furthermore, in his argument for God's free will in ST, Aquinas explicitly draws the distinction which raises the worry to which I have called attention here. He says,

‘Since God wills his own goodness of necessity but other things not of necessity… with respect to those things which he wills not out of necessity, he has liberum arbitrium’.²⁵

Here, then, Aquinas distinguishes between acts of will necessary for God, such as the will for his own goodness, and acts of will not necessary for God, such as the act of will to create. How is this distinction not a real distinction in God? Furthermore, it seems, on the face of it, that this analysis attributes contingency to some of God's acts, such as God's act of will to create. But if some divine acts are contingent, then it seems that God does have accidental properties, properties such that God could exist and have properties other than these, contrary to the explicit claims of the doctrine of divine simplicity, as Aquinas expounds and defends it. The central claims of the doctrine of divine simplicity as Aquinas understands it thus still seem falsified by Aquinas's account of free choice in God.

God's Accidental Properties

It helps considerably in this connection to look more closely at Aquinas's understanding of the nature of accidental properties.

The first thing to notice here is that although Aquinas denies that there are any accidental properties in God,²⁶ he also claims that it is possible for God to do things he does not do (possit facere quae non facit). So, for example, in a passage that deserves quoting at length, Aquinas says,

‘Some have supposed that God acts as it were from the necessity of nature, … in such a way that from the divine operation there can result neither other things nor another order of things, except what is now. But we have shown… that God does not act as it were from the necessity of nature but that his will is the cause of all things and also that his will is not naturally or of necessity determined to these things. And so in no way is it the case that this course of things comes from God of necessity in such a way that other things could not come [instead]. Others, however, have said that the divine power is determined to this course of things because of the order of divine wisdom and justice, without which God does nothing. But since the power of God, which is his essence, is nothing other than the wisdom of God, it can appropriately be said that there is nothing in the power of God which is not in the order of the divine wisdom, for the divine wisdom embraces the whole potency of [divine] power. And yet the order introduced in things by the divine wisdom, in which the formula of justice consists, … does not exhaust (non adaequat) the divine wisdom, so that the divine wisdom is limited to this order. … And so we must say unconditionally that God is able to do things other than those he does’.²⁷

Elsewhere he says,

‘although God does not will to do other than he does, he can will other things; and so, speaking unconditionally (absolute), he can do other things [than he does]’.²⁸

Aquinas emphasizes this point in speaking of God's liberum arbitrium or free choice. God creates freely, on Aquinas's view, and the freedom at issue in God's willing of creation, unlike God's willing of his own goodness, does involve alternative possibilities. Aquinas says,

‘the divine will is related to opposites, not in such a way that he wills something and afterwards wills it not [to be], which would be incompatible with his immutability, and not that he is able to will good and evil, because [this] would suppose defect in God, but because he is able to will or not to will this’.²⁹

So, on Aquinas's view, in this world God wills, for example, to create, but it is not necessary that God create; it is possible that God not create.³⁰ There is therefore another possible world in which God exists and does not will to create.

Thomists have typically supposed that Aquinas's claim that God has no accidents is consistent with his claim that God could do other than he does.³¹ But how are these positions to be reconciled? If God can do other than he does, then it is possible for God to exist as God and yet will differently from the way he actually does will. In that case, however, on our current way of thinking about modality, the way God actually wills is not necessary to him. Hence, that God wills in the way he does is a contingent fact about God and God's willing in this way is an accident of his. And yet Aquinas holds not only that God has no accidents but even that God is his own nature; and so, since the nature of God is invariable, it seems that God must be the same in all possible worlds in which he exists.

In my view, it is unreasonable to suppose that Aquinas is guilty of a large, explicit, obvious, and uncomplicated contradiction. A more reasonable explanation of this apparent conflict in his views is therefore that Aquinas's modal claims do not mean what current notions of modality would take them to mean. In what follows, for the sake of brevity, I will concentrate just on the problems generated by Aquinas's denial that God has accidents.

When Aquinas himself describes an accident, he does not categorize it as a property a thing does not have at a time but could have had at that time (or has at that time but could have not had at that time), or in any other way that suggests he is thinking of accidents in terms of synchronous possibilities across different possible worlds. Instead, he characterizes an accident entirely differently, namely, as something that has being but in an incomplete sort of way.³² So, for example, he says,

‘because essence is what is signified by a definition, it must be the case that [accidents] have essence in the same way in which they have a definition. But they have an incomplete definition, because they cannot be defined unless a subject is put in their definition. And the reason for this is that they do not have being per se, devoid of a subject. Instead, as substantial being results from form and matter when they come together in composition, so accidental being results from a subject and an accident when an accident comes to a subject. ….That to which an accident comes is a being complete in itself, subsisting in its own being, which naturally precedes the accident which comes to it. And so the conjunction of the accident coming [to a subject] with that to which the accident comes does not cause that being in which a thing subsists, by means of which a thing is a being per se, but it causes a certain kind of secondary being, without which a subsistent thing can be understood to be…. And so from an accident and a subject is not produced something that is one per se but only [something that is] one per accidens’.³³

It is clear from this passage that Aquinas does not characterize an accident as any property a thing has in some but not all of the possible worlds in which it exists, so that every feature a thing fails to have in all the worlds in which it exists has to count as an accident.

But doesn't it seem as if Aquinas should have defined an accident in just this way? Of the ten Aristotelian categories, all nine other than substance count as accidents; and the category of substance is the category including individuals and their essences. So anything non-essential to a particular thing is in fact an accident. And since the essence of a thing is the same in every possible world in which it exists, any feature a thing has in some but not all the possible worlds in which it exists will apparently have to be an accident, even on Aquinas's metaphysics. It seems, then, that on Aquinas's own views an accident must be any feature of a thing which that thing could have but doesn't have to have – that is, any feature which a thing has in some but not all the possible worlds in which it exists.

If this were Aquinas's position, however, then he could hardly maintain that God has no accidents but that God could do other than he does.

In the passage cited above, Aquinas's account of accidents emphasizes the metaphysical incompleteness of accidents, and it may be that this emphasis points us in the right direction for understanding his position about accidents and God. On Aquinas's view, an accident is what has only incomplete being. It doesn't have subsistent being or being per se, and its addition to something produces only an accidental unity, and not unity of the sort that is produced by the conjunction of matter and substantial form, which produces a substance. An ordinary created thing can be other than it is just in those parts of it that are metaphysically insubstantial, so to speak. A created thing is metaphysically limited enough that only some of its metaphysical parts, its invariant or necessary metaphysical parts, have complete being; and so its variant features, those it has in some but not all the possible parts in which it exists, have the sort of incomplete being Aquinas attributes to accidents. That is why, for created things categorized by means of the Aristotelian categories, it is true that any feature a thing has in some but not all the possible worlds in which it exists will be an accident. God, on the other hand, is metaphysically perfect and unlimited. And so, in the case of God, Aquinas seems to be thinking, even what is variable about him across possible worlds (as distinct from across time) has complete being.

If this is right, then this is the sense in which we should understand that God has no accidents – not that God is exactly the same in all possible worlds in which he exists but that there is nothing at all metaphysically incomplete or insubstantial about God in any respect, even though God is not the same in all possible worlds.

These remarks about Aquinas's understanding of the modal terms at issue in the doctrine of simplicity are only allusive and suggestive, not precise or analytically explanatory. But that they are roughly on the right track is further confirmed by the way Aquinas argues to the conclusion that God has no accidents. So, for example, as a quick and supposedly decisive argument in the sed contra of the relevant question in QDP, Aquinas says this:

‘every accident is dependent on something else (habet dependentiam ab alio). But there can be nothing of this sort in God, because anything that depends on something else must be caused, but God is the first cause [and] in no way caused [himself]’.³⁴

If by denying accidents of God Aquinas were trying to argue, in effect, that God is the same in all possible worlds, then it isn't at all clear that God's non-dependence would count as an acceptable argument for it, even on Aquinas's own views, since, as we have seen, Aquinas argues in various places that (non-dependent) God can do other than he does. The inference from non-dependence to sameness across possible worlds doesn't hold for Aquinas.

There is additional confirmation for this way of understanding Aquinas's concept of accidents in the reply Aquinas himself makes to a putative objector who raises the very sort of worry which has been at issue here. The objector says,

‘What is not necessary to be is equivalent to what is possible not to be. Therefore, if it is not necessary that God will something of the things that he wills, it is possible that he not will this, and it is [also] possible that he will that which he does not will. And so the divine will is contingent with respect to either of these. And in this way it is [also] imperfect, because everything contingent is imperfect and mutable’.³⁵

In his reply, Aquinas does not deny that God can will other than he does; he denies only that God's ability to will otherwise than he does entails that there is anything in God which is imperfect or changeable over time. He says,

‘sometimes a necessary cause has a non-necessary relation to an effect, and this is because of a defect in the effect, and not because of a defect in the cause…. That God does not will of necessity something of the things he wills happens not on account of a defect in the divine will but on account of a defect which belongs to what is willed in accordance with its formula (ratio), namely, because it is such that the perfect goodness of God can be without it. And this is a defect which accompanies every created good’.³⁶

I am glad to say that it is beyond the scope of this paper to give with any depth or precision a positive account of Aquinas's understanding of the notion of having an accident, but the evidence amassed here is enough to show clearly that Aquinas does not understand the concept of an accident as it would be understood in contemporary philosophy. In particular, he does not automatically take any property anything has in some but not all possible worlds in which it exists as an accident of that thing. However exactly Aquinas does understand the notion of having an accident, it is clear, then, that for him the denial that God has accidents does not entail that God is the same in all possible worlds in which he exists.

Conditional Necessity

Nonetheless, a problem remains. Even if we grant Aquinas more than many readers at this stage will be ready to concede, namely, that the claim that God can do other than he does can be reconciled with Aquinas's claim that God has no accidents, there is still some question whether Aquinas is entitled to hold, as he does, that all composition in God is ruled out. On the face of it, it seems that some composition must remain. That is because we can make a distinction – an apparently real distinction among intrinsic characteristics of God – between those divine acts of will that are the same in all possible worlds and those that vary across possible worlds. If we can make such a distinction, then it seems that God cannot be simple. In my view, this is the most difficult form of the objection that divine simplicity and divine free choice are incompatible.

The problem is that the distinction seems, on the face of it, to be a real distinction in God's nature, between the metaphysical ‘softness’ of willing to create (for example) and the metaphysical ‘hardness’ of willing goodness.³⁷ Willing to create characterizes God's nature in only some possible worlds, while willing goodness characterizes it in all possible worlds; therefore, it seems that there are at least two different sorts of characteristics in the divine nature, distinguished from one another by having or lacking the characteristic of obtaining in all possible worlds.

Aquinas, I think, would have supposed that this line of thought confuses a logical distinction to which we have every right with a metaphysical distinction for which there is no basis. On Aquinas's account of God's will, God wills himself and everything else he wills in a single simple act of will. Because some but not all of the objects of that single act of will might have been other than they are, we are warranted in drawing a logical distinction between the conditionally and the absolutely necessitated objects of that single act of will; but nothing in that warrant licenses the claim that the act of will is not entirely one, that there are two really distinct acts of will, or one act of will in two really distinct parts. Even if we should go so far as to say that with regard to some but not all of its objects God's will itself might have been different from what it is, this counterfactual claim shows us again only a logical distinction among the objects of the willing and not a metaphysical difference within the divine will itself. What the logical distinction picks out is a difference in the ways in which the single act of divine will is related to the divine nature, on the one hand, and to created things, on the other. But the mere fact that one thing is related in different ways to different things does not entail that it has distinct intrinsic properties, only distinct Cambridge properties. The difference between the relationship of the divine will to the divine nature and the relationship of the divine will to creatures stems not from a metaphysical difference in the divine will itself but from metaphysical differences among the diverse objects of that will.

An analogy may help clarify this part of Aquinas's position, even though it is fully suitable in only a few respects. If some woman, Monica, looks directly into a normal unobstructed mirror, then in a single glance she sees herself and other things. On any such occasion, Monica invariably sees herself, so that in the context of the example her seeing of herself is necessitated in some sense. But what she sees besides herself will vary from context to context and so is not necessitated. We might therefore draw a warrantable logical distinction between the necessitated seeing of herself and the non-necessitated seeing of other things. Still, that logical distinction provides no basis for inferring that there is a real distinction within Monica's act of seeing. Her act of seeing remains a single undivided glance in spite of its being properly subjected to our logical distinction. The basis for the logical distinction is not some division within Monica's glance but is rather the difference among the objects of her glance and the different ways in which those objects are related to Monica's one undifferentiated act of seeing.

If this line of thought is right, then Aquinas has all he wants or needs with regard to God's single act of will and its differing objects. The fact that we can distinguish conditionally from absolutely necessitated aspects of God's will shows us an appropriate logical distinction but provides no basis on which to infer a metaphysical distinction within the divine will itself. There is a necessary relationship between God's willing and God's nature considered as an object of his willing because his will is by definition a wanting of the good and God's nature is goodness, on Aquinas's view. But any other things God wills for the sake of goodness are such that goodness is realizable without them, and so the connection between God's will and these objects of his will is not necessary. Therefore, the distinction between those aspects of the divine will which could have been and those which could not have been otherwise reflects a difference in the ways in which the divine will is related to itself and to other things. And these different relationships give rise to different counterfactual truths – e.g. ‘God might have willed not to create'; ‘Even if God had not willed to create, he would still have willed himself’. But although the differing relationships and differing counterfactuals imply that God is not the same in all possible worlds, they do not show that in any given world God's act of will is not one single metaphysically indivisible act. They provide the basis for drawing a conceptual distinction among Cambridge properties of God's will, but because the distinction arises just from considering the different ways in which the divine will can be related to its objects, they do not constitute a metaphysical distinction among God's intrinsic properties any more than Monica's single glance is intrinsically divisible because of the different sorts of objects to which it is related. But absolute simplicity rules out only metaphysical differences within God's nature; it does not and could not provide any basis for objecting to logical or conceptual differences. And so the conceptual distinction between those aspects of the divine nature which could have been otherwise and those which could not is compatible with the doctrine of simplicity.

Conclusion

Aquinas's central metaethical thesis, worked out in the context of his metaphysics of the transcendentals, provides a sophisticated grounding for his virtue-based ethics. It constitutes, as it were, a grand unified theory of goodness, within which his account of human morality is situated, as a particular application of the general theory. When the central metaethical thesis is combined with Aquinas's theological views, especially his understanding of the doctrine of divine simplicity, then the theological interpretation of the central metaethical thesis constitutes the basis for a religious ethics that makes God essential to human morality but without tying morality to God's will. The result is a metaphysically grounded, objective virtue ethics which is theological at least in this sense that it is ultimately based in God's nature. But the success of this way of grounding and understanding ethics depends on Aquinas's interpretation of the doctrine of God's simplicity, which has seemed to many scholars to be the least acceptable part of Aquinas's philosophical theology. In this paper, I have tried to show that some of the problems apparently raised by the doctrine of simplicity stem from misunderstandings of the claims Aquinas takes to be constitutive of the doctrine. When those claims are properly understood, and in particular when they are understood in terms of Aquinas's own metaphysics and modal theory, they look considerably less open to attack. Although a full defense of the doctrine would require considerably more work on Aquinas's metaphysics and modal theory, the considerations I have raised here go some way to dispel some of the common objections to the doctrine.³⁸