Aquinas' ideas about the will are a complex of three powers of the human soul, which can be described as the intellect (perceptive, apprehensive, cognitive), the will (motive, appetitive, conative) and the passions or feelings (sensitive, emotive).
Eleonore Stump, in her 2003 book Aquinas, compared Aquinas' view of human freedom with contemporary accounts of free will. For Aquinas, she says, freedom is a property of the whole human being, not a component part of a person. Secondly, the will is not independent of the intellect. Intellect and will are engaged in a dynamic, complex interaction, with multiple stages between an initial perception and cognition by the intellect to the final action of the will, with occasional interruptions or overrides by the passions. All these stages may happen in the "twinkle of an eye" or in a long drawn-out process. At any stage the will can change the subject and ask the intellect to think about something else. The intellect may or may not do so, however. The whole picture greatly resembles the modern Theater of Consciousness model of Bernard Baars, with the will focusing or selectively directing the attention of the mind. (Also compare Ayn Rand and Richard Franklin.)
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Stump condenses Aquinas complicated picture of what goes on in an action to these simplified five stages.
One of these five stages, the electio, is most often identified with the liberum arbitrium - free decision or judgment. Aquinas used this term rather than free will (libera voluntas). Stump says that Aquinas puts limits on the liberum arbitrium and distinguishes freedom of action (limited by external constraints) from freedom of willing. He also recognizes that mental problems can cause a loss of intellect and so be an internal constraint on the will. Note that Aquinas locates multiple open possibilities or potentialities in the intellect. He locates the determination of the choice (the electio) in the will, which then causes the body to act. This agrees well with our Cogito model. But note that in Stump's fourth stage, it is the intellect that de-liberates, when it evaluates the best of the alternative means. Aquinas associates the liberum arbitrium with the power to do otherwise. In those rare cases when the intellect provides only a single means to achieve an end, the Counsel and Electio stages are skipped, but Aquinas nevertheless describes them as freely willed, even in the absence of alternative possibilities. This, says Stump, is like the contemporary debate about whether actions can be free in the absence of alternative possibilities. (See Harry Frankfurt, David Widerker.)
Following Aristotle, Aquinas in De Malo denied that the will was necessitated. That would be heretical because it eliminates praise and blame, denying moral responsibility.
On the other hand, for Aquinas' God, time is an eternal moment (totem simul). With respect to God's foreknowledge, Aquinas is a compatibilist. But Stump is convincing in her arguments that Aquinas may have been an incompatibilist with respect to causes, at least causes originating outside our minds. (See Peter van Inwagen's Consequence Argument.)
However, if the will was caused by events that originate within us (Aristotle's ἐν ἡμῖν), then Stump thinks Aquinas would regard those actions as free, because they would be "up to us" (Aristotle's ἐφ ἡμῖν).
Aquinas followed Aristotle in allowing chance events in the universe as accidentally intersecting causal chains. God as divine providence was seen as executing a plan (ratio) that includes continued governance of the world. Since providence means foresight, Aquinas believed in divine foreknowledge of all events, even those "chance" events.
(From Introduction to St. Thomas Aquinas, Modern Library, 1948, pp.368-375) Question LXXXIII of the Summa Theologica, in four articles We now inquire concerning free choice. Under this head there are four points of inquiry:
(1) Whether man has free choice?
First Article WHETHER MAN HAS FREE CHOICE? We proceed thus to the First Article: - Objection 1. It would seem that man has not free choice. For whoever has free choice does what he wills. But man does not what he wills, for it is written (Rom. vii. 19): For the good which I will I do not, but the evil which I will not, that I do. Therefore man has not free choice. Obj. 2. Further, whoever has free choice has in his power to will or not to will, to do or not to do. But this is not in man's power, for it is written (Rom. ix. 16): It is not of him that willeth — namely, to will — nor of him that runneth - namely, to run. Therefore man has not free choice. Obj. 3. Further, he is free who is his own master, as the Philosopher says. Therefore what is moved by another is not free. But God moves the will, for it is written (Prov. xxi. I): The heart of the king is in the hand of the Lord; whithersoever He will He shall turn it; and (Phil. ii. 13): It is God Who worketh in you both to will and to accomplish. Therefore man has not free choice. Obj. 4. Further, whoever has free choice is master of his own actions. But man is not master of his own actions, for it is written (Jer. X. 23) : The way of a man is not his, neither is it in a man to walk. Therefore man has not free choice. Obj. 5. Further, the Philosopher says: According as each one is, such does the end seem to him . But it is not in our power to be such as we are, for this comes to us from nature. Therefore it is natural to us to follow some particular end, and therefore we are not free in so doing. On the contrary, It is written (Ecclus. xv. W:) God made man from the beginning, and left him in the hand of his own counsel; and the Gloss adds: That is, in the liberty of choice. I answer that, Man has free choice, or otherwise counsels, exhortations, commands, prohibitions, rewards and punishments would be in vain. In order to make this evident, we must observe that some things act without judgment, as a stone moves downwards; and in like manner all things which lack knowledge. And some act from judgment, but not a free judgment; as brute animals. For the sheep, seeing the wolf, judges it a thing to be shunned, from a natural and not a free judgment; because it judges, not from deliberation, but from natural instinct. And the same thing is to be said of any judgment in brute animals. But man acts from judgment, because by his apprehensive power he judges that something should be avoided or sought. But because this judgment, in the case of some particular act, is not from a natural instinct, but from some act of comparison in the reason, therefore he acts from free judgment and retains the power of being inclined to various things. For reason in contingent matters may follow opposite courses, as we see in dialectical syllogisms and rhetorical arguments. Now particular operations are contingent, and therefore in such matters the judgment of reason may follow opposite courses, and is not determinate to one. And in that man is rational, it is necessary that he have free choice. Reply Obj. 1. As we have said above, the sensitive appetite, though it obeys the reason, yet in a given case can resist by desiring what the reason forbids. This is therefore the good which man does not when he wishes — namely, not to desire against reason, as Augustine says. Reply Obj. 2. Those words of the Apostle are not to be taken as though man does not wish or does not run of his free choice, but because free choice is not sufficient thereto unless it be moved and helped by God. Reply Obj. 3. Free choice is the cause of its own movement, because by his free choice man moves himself to act. But it does not of necessity belong to liberty that what is free should be the first cause of itself, as neither for one thing to be cause of another need it be the first cause. God, therefore, is the first cause, Who moves causes both natural and voluntary. And just as by moving natural causes He does not prevent their actions from being natural, so by moving voluntary causes He does not deprive their actions of being voluntary: but rather is He the cause of this very thing in them, for He operates in each thing according to its own nature. Reply Obj. 4. Man's way is said not to be his in the execution of his choice, wherein he may be impeded, whether he will or not. The choice itself, however, is in us, but presupposes the help of God. Reply Obj. 5. Quality in man is of two kinds: natural and adventitious. Now the natural quality may be in the intellectual part, or in the body and its powers. From the very fact, therefore, that man is such by virtue of a natural quality which is in the intellectual part, he naturally desires his last end, which is happiness. This desire is, indeed, a natural desire, and is not subject to free choice, as is clear from what we have said above. But on the part of the body and its powers, man may be such by virtue of a natural quality, inasmuch as he is of such a temperament or disposition due to any impression whatever produced by corporeal causes, which cannot affect the intellectual part, since it is not the act of a corporeal organ. And such as a man is by virtue of a corporeal quality, such also does his end seem to him, because from such a disposition a man is inclined to choose or reject something. But these inclinations are subject to the judgment of reason, which the lower appetite obeys, as we have said. Therefore this is in no way prejudicial to free choice. The adventitious qualities are habits and passions, by virtue of which a man is inclined to one thing rather than to another. And yet even these inclinations are subject to the judgment of reason. Such qualities, too, are subject to reason, as it is in our power either to acquire them, whether by causing them or disposing ourselves to them, or to reject them. And so there is nothing in this that is repugnant to free choice.
Second Article WHETHER FREE CHOICE IS A POWER? We proceed thus to the Second Article: Objection 1. It would seem that free choice is not a power, For free choice is nothing but a free judgment. But judgment denominates an act, not a power. Therefore free choice is not a power. Obj. 2. Further, free choice is defined as the faculty of the will and reason. But faculty denominates the facility of power, which is due to a habit. Therefore free choice is a habit. Moreover Bernard says that free choice is the soul's habit of disposing of itself. Therefore it is not a power. Obj. 3. Further, no natural power is forfeited through sin. But free choice is forfeited through sin, for Augustine says that man, by abusing free choice, loses both it and himself. Therefore free choice is not a power. On the contrary, Nothing but a power, seemingly, is the subject of a habit. But free choice is the subject of grace, by the help of which it chooses what is good. Therefore free choice is a power. I answer that, Although free choice, in its strict sense, denotes an act, in the common manner of speaking we call free choice that which is the principle of the act by which man judges freely. Now in us the principle of an act is both power and habit; for we say that we know something both by science and by the intellectual power. Therefore free choice must be either a power, or a habit, or a power with a habit. That it is neither a habit nor a power together with a habit can be clearly proved in two ways. First of all, because, if it is a habit, it must be a natural habit; for it is natural to man to have free choice. But there is no natural habit in us with respect to those things which come under free choice, for we are naturally inclined to those things of which we have natural habits, for instance, to assent to first principles. Now those things to which we are naturally inclined are not subject to free choice, as we have said in the, case of the desire of happiness. Therefore it is against the very notion of free choice that it should be a natural habit; and that it should be a non-natural habit is against its nature. Therefore in no sense is it a habit. Secondly, this is clear because habits are defined as that by reason of which we are well or ill disposed with regard to actions and passions. For by temperance we are well-disposed as regards concupiscences, and by intemperance ill-disposed; and by science we are well-disposed to the act of the intellect when we know the truth, and by the contrary habit ill-disposed. But free choice is indifferent to choosing well or ill, and therefore it is impossible that it be a habit. Therefore it is a power. Reply Obj. 1. It is not unusual for a power to be named from its act. And so from this act, which is a free judgment, is named the power which is the principle of this act. Otherwise, if free choice denominated an act, it would not always remain in man. Reply Obj. 2. Faculty sometimes denominates a power ready for operation, and in this sense faculty is used in the definition of free choice. But Bernard takes habit, not as divided against power, but as signifying any aptitude by which a man is somehow disposed to an act. This may be both by a power and by a habit, for by a power man is, as it were, empowered to do the action, and by the habit he is apt to act well or ill. Reply Obj. 3. Man is said to have lost free choice by falling into sin, not as to natural liberty, which is freedom from coercion, but as regards freedom from fault and unhappiness. Of this we shall treat later in the treatise on Morals in the second part of this work.
Third Article WHETHER FREE CHOICE IS AN APPETITIVE POWER? We proceed thus to the Third Article: Objection 1. It would seem that free choice is not an appetitive, but a cognitive power. For Damascene says that free choice straightway accompanies the rational power. But reason is a cognitive power. Therefore free choice is a cognitive power. Obj. 2. Further, free choice is so called as though it were a free judgment. But to judge is an act of a cognitive power. Therefore free choice is a cognitive power. Obj. 3. Further, the principal function of free choice is election. But election seems to belong to knowledge, because it implies a certain comparison of one thing to another; which belongs to the cognitive power. Therefore free choice is a cognitive power. On the contrary, The Philosopher says that election is the desire of those things which are in our power. But desire is an act of the appetitive power. Therefore election is also. But free choice is that by which we elect. Therefore free choice is an appetitive power. I answer that, The proper act of free choice is election, for we say that we have a free choice because we can take one thing while refusing another; and this is to elect. Therefore we must consider the nature of free choice by considering the nature of election. Now two things concur in election: one on the part of the cognitive power, the other on the part of the appetitive power. On the part of the cognitive power, counsel is required, by which we judge one thing to be preferred to another; on the part of the appetitive power, it is required that the appetite should accept the judgment of counsel. Therefore Aristotle leaves it in doubt whether election belongs principally to the appetitive or the cognitive power: since he says that election is either an appetitive intellect or an intellectual appetite. But he inclines to its being an intellectual appetite when he describes election as a desire proceeding from counsel. And the reason of this is because the proper object of election is the means to the end. Now the means, as such, has the nature of that good which is called useful; and since the good, as such, is the object of the appetite, it follows that election is principally an act of an appetitive power. And thus free choice is an appetitive power. Reply Obj. 1. The appetitive powers accompany the apprehensive, and in this sense Damascene says that free choice straightway accompanies the rational power. Reply Obj. 2. Judgment, as it were, concludes and terminates counsel. Now counsel is terminated, first, by the judgment of reason; secondly, by the acceptation of the appetite. Hence the Philosopher says that, having formed a judgment by counsel, we desire in accordance with that counsel.And in this sense election itself is a judgment from which free choice takes its name. Reply Obj. 3. This comparison which is implied in the term election belongs to the preceding counsel, which is an act of reason. For though the appetite does not make comparisons, yet inasmuch as it is moved by the apprehensive power which does compare, it has some likeness of comparison, by choosing one in preference to another.
Fourth Article WHETHER FREE CHOICE IS A POWER DISTINCT FROM THE WILL? We proceed thus to the Fourth Article: Objection 1. It would seem that free choice is a power distinct from the Will. For Damascene says that θέλησις is one thing and βούλησις another. But θέλησις is will, while βούλησις seems to be free choice, because βούλησις, according to him, is the will as concerning an object by way of comparison between two things. Therefore it seems that free choice is a power distinct from the will. Obj. 2. Further, powers are known by their acts. But election, which is the act of free choice, is distinct from the will, because the will regards the end, whereas choice regards the means to the end. Therefore free choice is a power distinct from the will. Obj. 3. Further, the will is the intellectual appetite. But on the part of the intellect there are two powers—agent and possible. Therefore, also on the part of the intellectual appetite there must be another power besides the will. And this, seemingly, can be only free choice. Therefore free choice is a power distinct from the will. On the contrary, Damascene says free choice is nothing else than the will. I answer that, The appetitive powers must be proportionate to the apprehensive powers, as we have said above. Now, as on the part of intellectual apprehension we have intellect and reason, so on the part of the intellectual appetite we have will and free choice, which is nothing else but the power of election. And this is clear from their relations to their respective objects and acts. For the act of understanding implies the simple acceptation of something, and hence we say that we understand first principles, which are known of themselves without any comparison. But to reason, properly speaking, is to come from one thing to the knowledge of another, and so, properly speaking, we reason about conclusions, which are known from the principles. In like manner, on the part of the appetite, to will implies the simple appetite for something, and so the will is said to regard the end, which is desired for itself. But to elect is to desire something for the sake of obtaining something else, and so, properly speaking, it regards the means to the end. Now in appetitive matters, the end is related to the means, which is desired for the end, in the same way as, in knowledge, principles are related to the conclusion to which we assent because of the principles. Therefore it is evident that as intellect is to reason, so will is to the elective power, which is free choice. But it has been shown above that it belongs to the same power both to understand and to reason even as it belongs to the same power to be at rest and to be in movement. Hence it belongs also to the same power to will and to elect. And on this account will and the free choice are not two powers, but one. Reply Obj. 1. βούλησις is distinct from θέλησις because of a distinction, not of powers, but of acts. Reply Obj. 2. Election and will — that is, the act of willing — are different acts, yet they belong to the same power, as do to understand and to reason, as we have said. Reply Obj. 3. The intellect is compared to the will as moving the will. And therefore there is no need to distinguish in the will an agent and a possible will.