Lecture 8 — The principle of movement in living beings: what it is not

Since the soul is defined in terms of two powers that belong to animate beings—the one, discernment, which is the work of intellect and sensation; the other, movement by local motion—and so much has been determined about sensation and understanding,

we must now contemplate what can be the moving factor in the soul: whether this is a single part of it, separable either in magnitude or by account, or the whole soul; and, if it is a part of the soul, whether it is a special part other than those generally acknowledged and already dealt with, or some one among these.

A difficulty at once arises as to how it is possible to speak at all of parts of the soul or to say how many they are. For in one way their number seems to be infinite and not merely, as some say, the reasoning, the irascible, and the appetitive; or as others say, the rational and irrational.

For according to the differences by which they separate these, other parts seem to show greater diversity than in those just mentioned: in particular, the vegetative part, which is in plants and all animals or living things; and the sensitive, which one cannot easily place among either the irrational or the rational elements.

Further, there is the imaginative power, which seems in its being to be quite different from any other. If one is to suppose that parts of the soul are separate, with which of these others it is identical, or from which it differs, are difficult problems.

Furthermore, there is the appetitive power, which seems distinct, both in account and as a different power, from all others; and it would be unsuitable to split this up. For will is in the rational part, desire and anger in the irrational; and if soul is three, an appetite will be in each.

But to come to the matter that is now before us: what is it that moves the animal by local motion? For the movement that is in all animals, by which they grow and decrease, would certainly seem to be due to the generative and vegetative power. Of respiration and exhalation, sleep and waking, we must treat later on: these also raise many difficulties.

But of local motion, we must consider what it is that moves an animal from place to place. It is obviously not the vegetative power. For a movement of this sort is always directed towards an end and is accompanied by phantasm or desire. For nothing is moved unless with desire or aversion, except under compulsion.

Further, plants would also move about and would possess some organic part suitable for such movement.

Likewise, it is not the sensitive power. Many animals, which have sensation, are fixed and motionless all their lives. Yet if nature is never purposeless and it never fails to provide what is necessary (save in incomplete or defective specimens, but animals of this sort here are complete and not defective, and there is a sign of this in that they reproduce, grow and decrease), it follows that they should also have the organic parts for moving about.

Nor is the reasoning power or what is called “intellect” the moving power; for the contemplative power does not contemplate in view of action and does not speak of the avoidable or the pursuable. But all movement is an avoidance of, or the pursuit of, something.

Nor, even when it does contemplate something of this kind, does it at once command pursuit or avoidance—for instance, it often understands something frightening or pleasant, but without commanding ourselves to fear, though the heart is moved, or, in the case of something pleasant, some other part.

Further, when the intellect and the understanding do command avoidance or pursuit, the soul does not move accordingly but acts according to desire, as in the case of one without self-control. And in general, we observe that one who knows the art of medicine does not necessarily heal—as though it were proper to some power other than science to act according to science.

But not even appetite has dominion over such movement as this. For those abstaining while still desiring and having an appetite for something do not do the things for which they have an appetite; instead, they obey reason.

795. Since the soul is defined by two powers (432a15). Having studied the soul in its vegetative, sensitive, and intellectual parts, the Philosopher now considers its fourth part, the motive part. And this is divided into two parts.

In the first part, he sets out his intention.

In the second, he pursues it, at a difficulty at once arises (432a22; [796]).

Introducing the problem, he observes first of all that even the old philosophers defined the soul in terms of two powers that belong to animate beings, that is, insofar as it has the power for two things, one of which is discernment, which arises from knowledge, which discernment is the work of the intellectual and sensitive parts; and the other is the power to move with local motion; and so much has been determined about sensation and understanding in what has preceded, now it is the moving principle that we must contemplate, what it is of the soul: whether it is a part of the soul separable from the other parts either in magnitude, that is, in subject, so that it is located in a place of the body distinct from the other powers, as the Platonists supposed, or, on the contrary, separable only in account; or whether it is not a part of the soul at all, but the entire soul; or whether, granted that it is a part, it is a part other than those usually acknowledged and distinct from those already studied in this treatise, or is it one of them.

796. Next, at a difficulty at once arises (432a22), he pursues his aim.

He first begins by way of disputation.

Second, he proceeds to a determination of certain conclusions, at it seems that these two are the movers (433a9; [818]).

This section contains two parts:

first, an argument against a certain division of the powers of the soul;

and second, an argument, at but of local motion (432b13; [808]), concerning especially the motive-principle as a part of the soul.

As regards the first, he does two things:

first, he states a division of the powers of the soul made by some philosophers;

and second, at for according to (432a26; [798]), he disputes it.

He begins then by saying that we are met at once in the beginning of our inquiry by the difficulty of how it is possible to distinguish the parts of the soul. What should be the principle of this division, and how many powers are there? In a way it seems to be infinite, that is, it seems that it cannot be comprehended under a precise number; and this would indeed be the truth if every act or movement in the soul had to be assigned to its own distinct part. Thus it seems that the parts are not as some determine it, that is, the rational, the irascible, and the appetitive (that is, concupiscible) part; for this division does not embrace all the parts of the soul but only the motive powers in man.

797. And others distinguish the soul’s powers into what has reason and what is non-rational, but this division too, though in a way it embraces all the parts of the soul, still is not a proper division of the parts of the soul as parts of the soul but only perhaps inasmuch as they are in the soul that has reason; and so Aristotle uses it in the Ethics. (Aristotle, Ethics 1.13, 1102a26–1103a3; cf. Aquinas, Commentary on Ethics, bk. 1, lect. 19–20.)

798. Next, at for according to the differences (432a26), he brings several objections against these divisions.

First, granted that the differences just mentioned between the various parts of the soul are sufficient to establish the diversity of the parts of the soul, one finds other parts that seem to show greater diversity than in those just mentioned. (Bk. 1, lect. 14.) For instance, in plants and all animals (and thus, in all living things) exists the vegetative part; and in all animals there is also the sensitive part; and these two obviously differ from each other, and also from the rational, irascible, and concupiscible parts, more than the irascible and concupiscible parts differ from each other. Yet neither of these two powers is included in the divisions given above.

799. Nor, it is clear, are they included in the first division: for it is obvious that neither the vegetative nor the sensitive can be said to be rational, irascible, or concupiscible. Whence, passing to the second division, he shows that it is not embraced under the second division, saying that it is not easy for anyone to place either the vegetative or the sensitive with either the irrational or rational elements.

800. That neither of these has reason is obvious, but that neither of these is irrational can become manifest from the following. For the irrational is either what is contrary to reason or what is naturally apt to have reason but does not have it; and neither of the aforesaid parts is such. (For if it were to signify only the negation of reason, it could not be presented as a genus of powers of soul.) Clearly, then, the aforesaid divisions of the soul’s powers are inadequate.

801. Then, at further, there is the imaginative power (432a31), he observes that the imaginative power of the soul, which can certainly be distinguished in its account from any other, has many difficult problems; is it identical with some of the powers already discussed, or does it differ, and especially if we were to follow those who say that the parts of the soul are separate in their subject.

802. Then again, at furthermore, there is the appetitive power (432b3), he remarks that the appetitive power seems distinct, both in account and as a different power, from all other parts of the soul. Now, assuming that the soul was so divided into a rational and an irrational part that each was a thing distinct from the other in subject, it will seem unsuitable to split up, that is, to divide, the appetitive part into two differing in subject. And this will be necessary if someone says the rational and irrational parts of the soul are distinct in subject; for there is something appetitive in the rational part (namely, the will) and something appetitive in the irrational part (namely, the irascible and concupiscible powers). And if someone distinguishes the parts of the soul into three distinct subjects (namely, the rational, the irascible, and the concupiscible), it will follow that an appetite will be in each of them: for will is in the rational part, as has been said, but the irascible is an appetite and so is the concupiscible; therefore, there will be three appetites in the soul, different in subject, according to the aforesaid division.

803. But the question now arises why the sensitive part of the soul has two appetitive capacities, the irascible and the concupiscible, whereas there is only one appetite in the rational, the will.

I answer that powers are distinguished according to the accounts of their objects; and the object of appetite is the good as apprehended.

804. Now, the intellect and the senses apprehend the good differently: the intellect apprehends it under the universal account of goodness, but the senses apprehend the good under a determinate account of goodness. And this is why there is only one kind of appetite following intellectual apprehension, and it is distinct from an appetite that follows sense apprehension; however, there are two appetites that follow the apprehension of sense, and they are distinguished according to the diverse accounts of apprehended good. For something apprehended by sense holds the account of the good desirable as delightful to the sense; and to this good is ordered the concupiscible appetite. But something can hold the account of the good and desirable insofar as it is put in charge of pleasant things, as itself having the capacity of freely using them; and to this is ordered the irascible appetite, which is what fights, as it were, on behalf of the concupiscible. Thus, animals only get angry and fight on account of things that are pleasant, that is to say, for the sake of food or mating, as it says in History of Animals 8. (Aristotle, History of Animals 8.1, 608b19–610a35.)

805. Therefore, all passions of the irascible appetite start from and end in passions of the concupiscible appetite. For anger springs from pain and ends in a pleasure; for the angry find their satisfaction in punishing. (Aristotle, Ethics 3.17, 1117a6–7.) Hence, some say that the difficult is the precise object of the irascible appetite.

806. But it is quite unreasonable to say, as some do, that the irascible appetite is ordered to avoiding evil. For one and the same power bears upon contraries—as sight upon both black and white; hence good and evil cannot by themselves differentiate appetitive powers. Hence, just as loving some good pertains to the concupiscible appetite, so does hating an evil, as Aristotle says in book two of the Topics; and similarly, both the hope of good and the fear of evil pertain to the irascible appetite. (Aristotle, Topics 2.7, 113a33–b3.)

807. Then, at but to come (432b7), he observes that what is now before us, namely, what is it that moves the animal by local motion, does not seem to be accounted for by the aforesaid divisions, because it does not seem to be contained under them. Now, concerning the movement of growth and decrease, which is common to all lower things, it is manifest that this has a principle common to all living things, that is, the generative and nutritive power. But the cause of certain other changes, such as breathing in and out, or sleeping and waking, should be determined later: they also raise many difficulties and must therefore be given a special treatment.

808. Then, at but of local motion (432b13), he asks disputatively what is the principle of local movement in animals.

And he shows first that it is not the vegetative power;

second, at likewise (432b19; [810]), that it is not a sensitive power;

third, at nor is the reasoning power (432b26; [812]), that it is not an intellective power;

and fourth, at but not even appetite (433a6; [817]), that it is not the appetitive power.

First, he says that now we must consider what it is that moves an animal progressively, according to place. That it is not the vegetative power he proves by two arguments.

The first is as follows. Progressive local movement from place to place is always directed towards an end the animal imagines and desires; for no animal is moved unless with desire or aversion for something, except perhaps by violence. But since the vegetative part is without imagination and desire, it cannot be the principle of this kind of local movement.

809. The second argument, at further, plants (432b17), runs thus. The vegetative part is also in plants; if, therefore, the vegetative part were the principle of progressive local motion, all plants would move about in this way and would possess the necessary organs for doing so: which they obviously do not and are not. Therefore, the vegetative part is not the principle of this kind of local movement.

810. Next, at likewise (432b19), he shows that sensation is not the principle we are seeking. For sense is in all animals; if, therefore, sense were the principle of the aforementioned motion, it would follow that every animal would be moved with this motion from place to place.

But this is false, since many animals, though capable of sensation, are fixed in one place and motionless so long as they live.

811. And to meet the objection that perhaps the reason why such animals are immobile is that they do not lack the motive principle but merely the organs suited for the movement, he adds that nature is never purposeless and it never fails to provide what is necessary for a given animal, unless this animal is incomplete or defective, as are deformed animals; these deformities in fact occur outside the intention of nature, due to a corruption of some principle within the parental seed. But immobile animals are complete in their own species, not defective like deformed births; a sign of this is that they generate offspring in their own likeness and they grow and decrease quite normally, which does not occur in defective animals. In such animals, therefore, nature does not act without purpose or fail in what is necessary for life.

If, then, they were endowed with a principle of motion, they would also have the organic parts of progressive motion; otherwise this motive principle in them would be superfluous, and they would be unequipped with the instruments necessary for the execution of the motive power. Note that this reasoning implies that wherever a principle of life exists, there also will be found organs congruent with that principle, and that the parts of a body are for the sake of the parts of its soul.

812. Next, where he says, nor is the reasoning power (432b26), he shows that not even the intellect is the motive principle, saying that nor is it the reasoning part, which is called “intellect,” that seems to be the moving power. Hence, we can understand here that reason and intellect are not different parts of the soul, but our very intellect is called “reason” insofar as it comes to know an intelligible truth by a certain inquiry.

813. That intellect is not the moving principle he first proves with regard to the speculative intellect. For the speculative intellect contemplatively considers things that can only be contemplated and cannot be brought about, for example, when one considers that a triangle has three angles equal to two right angles, and the like. Therefore it is manifest that the contemplative intellect does not contemplate in view of action, and does not speak of something avoidable or desirable. And in this way it cannot cause movement; for all movement is an avoidance of, or the pursuit of, something, by way of desire.

814. But sometimes the intellect considers something that can be done, yet not in a practical way but only speculatively—considering it universally, not insofar as it is a principle of a particular act. So he says that nor does the intellect, when it does contemplate, that is, consider speculatively, something of this kind, that is, something that can be brought about, right then command pursuit or avoidance. For example, we often think of something frightening or pleasant but the intellect is not commanding one to fear or desire them; but sometimes the heart is moved toward being afraid without a command of the intellect. And in the case of something pleasant, he adds, moving the appetite, there will be some other part that is moved besides the heart.

815. This he adds with Plato in mind, who located the parts of the soul in different parts of the body: the irascible part, whence fear arises, in the heart; and the concupiscible in some other part of the body, such as the liver. Clearly, then, the intellect when considering in a speculative way something that can be brought about does not move one. As such, it is evident the speculative intellect is in no way the motive power.

816. Next, at further, when (433a1), he shows that not even the practical intellect moves one to action. For he says that, even when the practical intellect does command, it is possible that when the intellect says something should be avoided or pursued, a man may nevertheless not be moved, but rather acts according to his desire—as is clear with those without self-control, who have correct reasoning but do not remain with their correct reason. It would seem, therefore, that intellect does not move us. Similarly, doctors who know the art of medicine do not recover health, because they do not do for themselves what their art commands. Apparently, then, to act according to science involves some other power besides even practical science.

817. Lastly, with but not even appetite (433a6), he shows that the appetitive part does not have dominion over our movements; for we may see people with self-control in a state of desire and concupiscence, yet they do not do the things for which they have an appetite. And the converse is true of those without self-control, as he explains more clearly in Ethics 7. (Aristotle, Ethics 7.1–10, 1145a15–1152a36; cf. Aquinas, Commentary on Ethics, bk. 7, lect. 3–4.) So it seems that neither is the appetite the cause of movement.