touch. But the intellectual soul has the power of sense in all its completeness; because what belongs to the inferior nature pre-exists more perfectly in the superior, as Dionysius says (Div. Nom. v).
Therefore the body to which the intellectual soul is united should be a mixed body, above others reduced to the most equable complexion. For this reason among animals, man has the best sense of touch. And among men, those who have the best sense of touch have the best intelligence. A sign of which is that we observe "those who are refined in body are well endowed in mind," as stated in _De Anima_ ii, 9.
Reply Obj. 1: Perhaps someone might attempt to answer this by saying that before sin the human body was incorruptible. This answer does not seem sufficient; because before sin the human body was immortal not by nature, but by a gift of Divine grace; otherwise its immortality would not be forfeited through sin, as neither was the immortality of the devil.
Therefore we answer otherwise by observing that in matter two conditions are to be found; one which is chosen in order that the matter be suitable to the form; the other which follows by force of the first disposition. The artisan, for instance, for the form of the saw chooses iron adapted for cutting through hard material; but that the teeth of the saw may become blunt and rusted, follows by force of the matter itself. So the intellectual soul requires a body of equable complexion, which, however, is corruptible by force of its matter. If, however, it be said that God could avoid this, we answer that in the formation of natural things we do not consider what God might do; but what is suitable to the nature of things, as Augustine says (Gen. ad lit. ii, 1). God, however, provided in this case by applying a remedy against death in the gift of grace.
Reply Obj. 2: A body is not necessary to the intellectual soul by reason of its intellectual operation considered as such; but on account of the sensitive power, which requires an organ of equable temperament. Therefore the intellectual soul had to be united to such a body, and not to a simple element, or to a mixed body, in which fire was in excess; because otherwise there could not be an equability of temperament. And this body of an equable temperament has a dignity of its own by reason of its being remote from contraries, thereby resembling in a way a heavenly body.
Reply Obj. 3: The parts of an animal, for instance, the eye, hand, flesh, and bones, and so forth, do not make the species; but the whole does, and therefore, properly speaking, we cannot say that these are of different species, but that they are of various dispositions. This is suitable to the intellectual soul, which, although it be one in its essence, yet on account of its perfection, is manifold in power: and therefore, for its various operations it requires various dispositions in the parts of the body to which it is united. For this reason we observe that there is a greater variety of parts in perfect than in imperfect animals; and in these a greater variety than in plants.
Reply Obj. 4: The intellectual soul as comprehending universals, has a power extending to the infinite; therefore it cannot be limited by nature to certain fixed natural notions, or even to certain fixed means whether of defence or of clothing, as is the case with other animals, the souls of which are endowed with knowledge and power in regard to fixed particular things. Instead of all these, man has by nature his reason and his hands, which are "the organs of organs" (De Anima iii), since by their means man can make for himself instruments of an infinite variety, and for any number of purposes.
Whether the Intellectual Soul Is United to the Body Through the Medium of Accidental Dispositions?
Objection 1: It would seem that the intellectual soul is united to the body through the medium of accidental dispositions. For every form exists in its proper disposed matter. But dispositions to a form are accidents. Therefore we must presuppose accidents to be in matter before the substantial form; and therefore before the soul, since the soul is a substantial form.
Obj. 2: Further, various forms of one species require various parts of matter. But various parts of matter are unintelligible without division in measurable quantities. Therefore we must suppose dimensions in matter before the substantial forms, which are many belonging to one species.
Obj. 3: Further, what is spiritual is connected with what is corporeal by virtual contact. But the virtue of the soul is its power. Therefore it seems that the soul is united to the body by means of a power, which is an accident.
_On the contrary,_ Accident is posterior to substance, both in the order of time and in the order of reason, as the Philosopher says, _Metaph._ vii (Did. vi, 1). Therefore it is unintelligible that any accidental form exist in matter before the soul, which is the substantial form.
_I answer that,_ If the soul were united to the body, merely as a motor, there would be nothing to prevent the existence of certain dispositions mediating between the soul and the body; on the contrary, they would be necessary, for on the part of the soul would be required the power to move the body; and on the part of the body, a certain aptitude to be moved by the soul.
If, however, the intellectual soul is united to the body as the substantial form, as we have already said above (A. 1), it is impossible for any accidental disposition to come between the body and the soul, or between any substantial form whatever and its matter. The reason is because since matter is in potentiality to all manner of acts in a certain order, what is absolutely first among the acts must be understood as being first in matter. Now the first among all acts is existence. Therefore, it is impossible for matter to be apprehended as hot, or as having quantity, before it is actual. But matter has actual existence by the substantial form, which makes it to exist absolutely, as we have said above (A. 4). Wherefore it is impossible for any accidental dispositions to pre-exist in matter before the substantial form, and consequently before the soul.
Reply Obj. 1: As appears from what has been already said (A. 4), the more perfect form virtually contains whatever belongs to the inferior forms; therefore while remaining one and the same, it perfects matter according to the various degrees of perfection. For the same essential form makes man an actual being, a body, a living being, an animal, and a man. Now it is clear that to every genus follow its own proper accidents. Therefore as matter is apprehended as perfected in its existence, before it is understood as corporeal, and so on; so those accidents which belong to existence are understood to exist before corporeity; and thus dispositions are understood in matter before the form, not as regards all its effects, but as regards the subsequent effect.
Reply Obj. 2: Dimensions of quantity are accidents consequent to the corporeity which belongs to the whole matter. Wherefore matter, once understood as corporeal and measurable, can be understood as distinct in its various parts, and as receptive of different forms according to the further degrees of perfection. For although it is essentially the same form which gives matter the various degrees of perfection, as we have said (ad 1), yet it is considered as different when brought under the observation of reason.
Reply Obj. 3: A spiritual substance which is united to a body as its motor only, is united thereto by power or virtue. But the intellectual soul is united by its very being to the body as a form; and yet it guides and moves the body by its power and virtue.
Whether the Soul Is United to the Animal Body by Means of a Body?
Objection 1: It seems that the soul is united to the animal body by means of a body. For Augustine says (Gen. ad lit. vii, 19), that "the soul administers the body by light," that is, by fire, "and by air, which is most akin to a spirit." But fire and air are bodies.
Therefore the soul is united to the human body by means of a body.
Obj. 2: Further, a link between two things seems to be that thing the removal of which involves the cessation of their union. But when breathing ceases, the soul is separated from the body. Therefore the breath, which is a subtle body, is the means of union between soul and body.
Obj. 3: Further, things which are very distant from one another, are not united except by something between them. But the intellectual soul is very distant from the body, both because it is incorporeal, and because it is incorruptible. Therefore it seems to be united to the body by means of an incorruptible body, and such would be some heavenly light, which would harmonize the elements, and unite them together.
_On the contrary,_ The Philosopher says (De Anima ii, 1): "We need not ask if the soul and body are one, as neither do we ask if wax and its shape are one." But the shape is united to the wax without a body intervening. Therefore also the soul is thus united to the body.
_I answer that,_ If the soul, according to the Platonists, were united to the body merely as a motor, it would be right to say that some other bodies must intervene between the soul and body of man, or any animal whatever; for a motor naturally moves what is distant from it by means of something nearer.
If, however, the soul is united to the body as its form, as we have said (A. 1), it is impossible for it to be united by means of another body. The reason of this is that a thing is one, according as it is a being. Now the form, through itself, makes a thing to be actual since it is itself essentially an act; nor does it give existence by means of something else. Wherefore the unity of a thing composed of matter and form, is by virtue of the form itself, which by reason of its very nature is united to matter as its act. Nor is there any other cause of union except the agent, which causes matter to be in act, as the Philosopher says, _Metaph._ viii (Did. vii, 6).
From this it is clear how false are the opinions of those who maintained the existence of some mediate bodies between the soul and body of man. Of these certain Platonists said that the intellectual soul has an incorruptible body naturally united to it, from which it is never separated, and by means of which it is united to the corruptible body of man. Others said that the soul is united to the body by means of a corporeal spirit. Others said it is united to the body by means of light, which, they say, is a body and of the nature of the fifth essence; so that the vegetative soul would be united to the body by means of the light of the sidereal heaven; the sensible soul, by means of the light of the crystal heaven; and the intellectual soul by means of the light of the empyrean heaven. Now all this is fictitious and ridiculous: for light is not a body; and the fifth essence does not enter materially into the composition of a mixed body (since it is unchangeable), but only virtually: and lastly, because the soul is immediately united to the body as the form to matter.
Reply Obj. 1: Augustine speaks there of the soul as it moves the body; whence he uses the word "administration." It is true that it moves the grosser parts of the body by the more subtle parts. And the first instrument of the motive power is a kind of spirit, as the Philosopher says in De causa motus animalium (De mot. animal. x).
Reply Obj. 2: The union of soul and body ceases at the cessation of breath, not because this is the means of union, but because of the removal of that disposition by which the body is disposed for such a union. Nevertheless the breath is a means of moving, as the first instrument of motion.
Reply Obj. 3: The soul is indeed very distant from the body, if we consider the condition of each separately: so that if each had a separate existence, many means of connection would have to intervene.
But inasmuch as the soul is the form of the body, it has not an existence apart from the existence of the body, but by its own existence is united to the body immediately. This is the case with every form which, if considered as an act, is very distant from matter, which is a being only in potentiality.
Whether the Soul Is in Each Part of the Body?
Objection 1: It would seem that the whole soul is not in each part of the body; for the Philosopher says in _De causa motus animalium_ (De mot. animal. x): "It is not necessary for the soul to be in each part of the body; it suffices that it be in some principle of the body causing the other parts to live, for each part has a natural movement of its own."
Obj. 2: Further, the soul is in the body of which it is the act. But it is the act of an organic body. Therefore it exists only in an organic body. But each part of the human body is not an organic body.
Therefore the whole soul is not in each part.
Obj. 3: Further, the Philosopher says (De Anima ii, 1) that the relation of a part of the soul to a part of the body, such as the sight to the pupil of the eye, is the same as the relation of the soul to the whole body of an animal. If, therefore, the whole soul is in each part of the body, it follows that each part of the body is an animal.
Obj. 4: Further, all the powers of the soul are rooted in the essence of the soul. If, therefore, the whole soul be in each part of the body, it follows that all the powers of the soul are in each part of the body; thus the sight will be in the ear, and hearing in the eye, and this is absurd.
Obj. 5: Further, if the whole soul is in each part of the body, each part of the body is immediately dependent on the soul. Thus one part would not depend on another; nor would one part be nobler than another; which is clearly untrue. Therefore the soul is not in each part of the body.
_On the contrary,_ Augustine says (De Trin. vi, 6), that "in each body the whole soul is in the whole body, and in each part is entire."
_I answer that,_ As we have said, if the soul were united to the body merely as its motor, we might say that it is not in each part of the body, but only in one part through which it would move the others. But since the soul is united to the body as its form, it must necessarily be in the whole body, and in each part thereof. For it is not an accidental form, but the substantial form of the body. Now the substantial form perfects not only the whole, but each part of the whole. For since a whole consists of parts, a form of the whole which does not give existence to each of the parts of the body, is a form consisting in composition and order, such as the form of a house; and such a form is accidental. But the soul is a substantial form; and therefore it must be the form and the act, not only of the whole, but also of each part. Therefore, on the withdrawal of the soul, as we do not speak of an animal or a man unless equivocally, as we speak of a painted animal or a stone animal; so is it with the hand, the eye, the flesh and bones, as the Philosopher says (De Anima ii, 1). A proof of which is, that on the withdrawal of the soul, no part of the body retains its proper action; although that which retains its species, retains the action of the species. But act is in that which it actuates: wherefore the soul must be in the whole body, and in each part thereof.
That it is entire in each part thereof, may be concluded from this, that since a whole is that which is divided into parts, there are three kinds of totality, corresponding to three kinds of division.
There is a whole which is divided into parts of quantity, as a whole line, or a whole body. There is also a whole which is divided into logical and essential parts: as a thing defined is divided into the parts of a definition, and a composite into matter and form. There is, further, a third kind of whole which is potential, divided into virtual parts. The first kind of totality does not apply to forms, except perhaps accidentally; and then only to those forms, which have an indifferent relationship to a quantitative whole and its parts; as whiteness, as far as its essence is concerned, is equally disposed to be in the whole surface and in each part of the surface; and, therefore, the surface being divided, the whiteness is accidentally divided. But a form which requires variety in the parts, such as a soul, and specially the soul of perfect animals, is not equally related to the whole and the parts: hence it is not divided accidentally when the whole is divided. So therefore quantitative totality cannot be attributed to the soul, either essentially or accidentally. But the second kind of totality, which depends on logical and essential perfection, properly and essentially belongs to forms: and likewise the virtual totality, because a form is the principle of operation.
Therefore if it be asked whether the whole whiteness is in the whole surface and in each part thereof, it is necessary to distinguish. If we mean quantitative totality which whiteness has accidentally, then the whole whiteness is not in each part of the surface. The same is to be said of totality of power: since the whiteness which is in the whole surface moves the sight more than the whiteness which is in a small part thereof. But if we mean totality of species and essence, then the whole whiteness is in each part of a surface.
Since, however, the soul has not quantitative totality, neither essentially, nor accidentally, as we have seen; it is enough to say that the whole soul is in each part of the body, by totality of perfection and of essence, but not by totality of power. For it is not in each part of the body, with regard to each of its powers; but with regard to sight, it is in the eye; and with regard to hearing, it is in the ear; and so forth. We must observe, however, that since the soul requires variety of parts, its relation to the whole is not the same as its relation to the parts; for to the whole it is compared primarily and essentially, as to its proper and proportionate perfectible; but to the parts, secondarily, inasmuch as they are ordained to the whole.
Reply Obj. 1: The Philosopher is speaking there of the motive power of the soul.
Reply Obj. 2: The soul is the act of an organic body, as of its primary and proportionate perfectible.
Reply Obj. 3: An animal is that which is composed of a soul and a whole body, which is the soul's primary and proportionate perfectible. Thus the soul is not in a part. Whence it does not follow that a part of an animal is an animal.
Reply Obj. 4: Some of the powers of the soul are in it according as it exceeds the entire capacity of the body, namely the intellect and the will; whence these powers are not said to be in any part of the body. Other powers are common to the soul and body; wherefore each of these powers need not be wherever the soul is, but only in that part of the body, which is adapted to the operation of such a power.
Reply Obj. 5: One part of the body is said to be nobler than another, on account of the various powers, of which the parts of the body are the organs. For that part which is the organ of a nobler power, is a nobler part of the body: as also is that part which serves the same power in a nobler manner.
QUESTION 77 OF THOSE THINGS WHICH BELONG TO THE POWERS OF THE SOUL IN GENERAL (In Eight Articles) We proceed to consider those things which belong to the powers of the soul; first, in general, secondly, in particular. Under the first head there are eight points of inquiry: (1) Whether the essence of the soul is its power?
(2) Whether there is one power of the soul, or several?
(3) How the powers of the soul are distinguished from one another?
(4) Of the orders of the powers, one to another; (5) Whether the powers of the soul are in it as in their subject?
(6) Whether the powers flow from the essence of the soul?
(7) Whether one power rises from another?
(8) Whether all the powers of the soul remain in the soul after death?
Whether the Essence of the Soul Is Its Power?
Objection 1: It would seem that the essence of the soul is its power.
For Augustine says (De Trin. ix, 4), that "mind, knowledge, and love are in the soul substantially, or, which is the same thing, essentially": and (De Trin. x, 11), that "memory, understanding, and will are one life, one mind, one essence."
Obj. 2: Further, the soul is nobler than primary matter. But primary matter is its own potentiality. Much more therefore is the soul its own power.
Obj. 3: Further, the substantial form is simpler than the accidental form; a sign of which is that the substantial form is not intensified or relaxed, but is indivisible. But the accidental form is its own power. Much more therefore is that substantial form which is the soul.
Obj. 4: Further, we sense by the sensitive power and we understand by the intellectual power. But "that by which we first sense and understand" is the soul, according to the Philosopher (De Anima ii, 2). Therefore the soul is its own power.
Obj. 5: Further, whatever does not belong to the essence is an accident. Therefore if the power of the soul is something else besides the essence thereof, it is an accident, which is contrary to Augustine, who says that the foregoing (see Obj. 1) "are not in the soul as in a subject as color or shape, or any other quality, or quantity, are in a body; for whatever is so, does not exceed the subject in which it is: Whereas the mind can love and know other things" (De Trin. ix, 4).
Obj. 6: Further, "a simple form cannot be a subject." But the soul is a simple form; since it is not composed of matter and form, as we have said above (Q. 75, A. 5). Therefore the power of the soul cannot be in it as in a subject.
Obj. 7: Further, an accident is not the principle of a substantial difference. But sensitive and rational are substantial differences; and they are taken from sense and reason, which are powers of the soul. Therefore the powers of the soul are not accidents; and so it would seem that the power of the soul is its own essence.
_On the contrary,_ Dionysius (Coel. Hier. xi) says that "heavenly spirits are divided into essence, power, and operation." Much more, then, in the soul is the essence distinct from the virtue or power.
_I answer that,_ It is impossible to admit that the power of the soul is its essence, although some have maintained it. For the present purpose this may be proved in two ways. First, because, since power and act divide being and every kind of being, we must refer a power and its act to the same genus. Therefore, if the act be not in the genus of substance, the power directed to that act cannot be in the genus of substance. Now the operation of the soul is not in the genus of substance; for this belongs to God alone, whose operation is His own substance. Wherefore the Divine power which is the principle of His operation is the Divine Essence itself. This cannot be true either of the soul, or of any creature; as we have said above when speaking of the angels (Q. 54, A. 3). Secondly, this may be also shown to be impossible in the soul. For the soul by its very essence is an act. Therefore if the very essence of the soul were the immediate principle of operation, whatever has a soul would always have actual vital actions, as that which has a soul is always an actually living thing. For as a form the soul is not an act ordained to a further act, but the ultimate term of generation. Wherefore, for it to be in potentiality to another act, does not belong to it according to its essence, as a form, but according to its power. So the soul itself, as the subject of its power, is called the first act, with a further relation to the second act. Now we observe that what has a soul is not always actual with respect to its vital operations; whence also it is said in the definition of the soul, that it is "the act of a body having life potentially"; which potentiality, however, "does not exclude the soul." Therefore it follows that the essence of the soul is not its power. For nothing is in potentiality by reason of an act, as act.
Reply Obj. 1: Augustine is speaking of the mind as it knows and loves itself. Thus knowledge and love as referred to the soul as known and loved, are substantially or essentially in the soul, for the very substance or essence of the soul is known and loved. In the same way are we to understand what he says in the other passage, that those things are "one life, one mind, one essence." Or, as some say, this passage is true in the sense in which the potential whole is predicated of its parts, being midway between the universal whole, and the integral whole. For the universal whole is in each part according to its entire essence and power; as animal in a man and in a horse; and therefore it is properly predicated of each part. But the integral whole is not in each part, neither according to its whole essence, nor according to its whole power. Therefore in no way can it be predicated of each part; yet in a way it is predicated, though improperly, of all the parts together; as if we were to say that the wall, roof, and foundations are a house. But the potential whole is in each part according to its whole essence, not, however, according to its whole power. Therefore in a way it can be predicated of each part, but not so properly as the universal whole. In this sense, Augustine says that the memory, understanding, and the will are the one essence of the soul.
Reply Obj. 2: The act to which primary matter is in potentiality is the substantial form. Therefore the potentiality of matter is nothing else but its essence.
Reply Obj. 3: Action belongs to the composite, as does existence; for to act belongs to what exists. Now the composite has substantial existence through the substantial form; and it operates by the power which results from the substantial form. Hence an active accidental form is to the substantial form of the agent (for instance, heat compared to the form of fire) as the power of the soul is to the soul.
Reply Obj. 4: That the accidental form is a principle of action is due to the substantial form. Therefore the substantial form is the first principle of action; but not the proximate principle. In this sense the Philosopher says that "the soul is that whereby we understand and sense."
Reply Obj. 5: If we take accident as meaning what is divided against substance, then there can be no medium between substance and accident; because they are divided by affirmation and negation, that is, according to existence in a subject, and non-existence in a subject. In this sense, as the power of the soul is not its essence, it must be an accident; and it belongs to the second species of accident, that of quality. But if we take accident as one of the five universals, in this sense there is a medium between substance and accident. For the substance is all that belongs to the essence of a thing; whereas whatever is beyond the essence of a thing cannot be called accident in this sense; but only what is not caused by the essential principle of the species. For the 'proper' does not belong to the essence of a thing, but is caused by the essential principles of the species; wherefore it is a medium between the essence and accident thus understood. In this sense the powers of the soul may be said to be a medium between substance and accident, as being natural properties of the soul. When Augustine says that knowledge and love are not in the soul as accidents in a subject, this must be understood in the sense given above, inasmuch as they are compared to the soul, not as loving and knowing, but as loved and known. His argument proceeds in this sense; for if love were in the soul loved as in a subject, it would follow that an accident transcends its subject, since even other things are loved through the soul.
Reply Obj. 6: Although the soul is not composed of matter and form, yet it has an admixture of potentiality, as we have said above (Q.
75, A. 5, ad 4); and for this reason it can be the subject of an accident. The statement quoted is verified in God, Who is the Pure Act; in treating of which subject Boethius employs that phrase (De Trin. i).
Reply Obj. 7: Rational and sensitive, as differences, are not taken from the powers of sense and reason, but from the sensitive and rational soul itself. But because substantial forms, which in themselves are unknown to us, are known by their accidents; nothing prevents us from sometimes substituting accidents for substantial differences.
Whether There Are Several Powers of the Soul?
Objection 1: It would seem that there are not several powers of the soul. For the intellectual soul approaches nearest to the likeness of God. But in God there is one simple power: and therefore also in the intellectual soul.
Obj. 2: Further, the higher a power is, the more unified it is. But the intellectual soul excels all other forms in power. Therefore above all others it has one virtue or power.
Obj. 3: Further, to operate belongs to what is in act. But by the one essence of the soul, man has actual existence in the different degrees of perfection, as we have seen above (Q. 76, AA. 3, 4).
Therefore by the one power of the soul he performs operations of various degrees.
_On the contrary,_ The Philosopher places several powers in the soul (De Anima ii, 2,3).
_I answer that,_ Of necessity we must place several powers in the soul. To make this evident, we observe that, as the Philosopher says (De Coelo ii, 12), the lowest order of things cannot acquire perfect goodness, but they acquire a certain imperfect goodness, by few movements; and those which belong to a higher order acquire perfect goodness by many movements; and those yet higher acquire perfect goodness by few movements; and the highest perfection is found in those things which acquire perfect goodness without any movement whatever. Thus he is least of all disposed of health, who can only acquire imperfect health by means of a few remedies; better disposed is he who can acquire perfect health by means of many remedies; and better still, he who can by few remedies; best of all is he who has perfect health without any remedies. We conclude, therefore, that things which are below man acquire a certain limited goodness; and so they have a few determinate operations and powers. But man can acquire universal and perfect goodness, because he can acquire beatitude. Yet he is in the last degree, according to his nature, of those to whom beatitude is possible; therefore the human soul requires many and various operations and powers. But to angels a smaller variety of powers is sufficient. In God there is no power or action beyond His own Essence.
There is yet another reason why the human soul abounds in a variety of powers--because it is on the confines of spiritual and corporeal creatures; and therefore the powers of both meet together in the soul.
Reply Obj. 1: The intellectual soul approaches to the Divine likeness, more than inferior creatures, in being able to acquire perfect goodness; although by many and various means; and in this it falls short of more perfect creatures.
Reply Obj. 2: A unified power is superior if it extends to equal things: but a multiform power is superior to it, if it is over many things.
Reply Obj. 3: One thing has one substantial existence, but may have several operations. So there is one essence of the soul, with several powers.
Whether the Powers Are Distinguished by Their Acts and Objects?
Objection 1: It would seem that the powers of the soul are not distinguished by acts and objects. For nothing is determined to its species by what is subsequent and extrinsic to it. But the act is subsequent to the power; and the object is extrinsic to it. Therefore the soul's powers are not specifically distinct by acts and objects.
Obj. 2: Further, contraries are what differ most from each other.
Therefore if the powers are distinguished by their objects, it follows that the same power could not have contrary objects. This is clearly false in almost all the powers; for the power of vision extends to white and black, and the power to taste to sweet and bitter.
Obj. 3: Further, if the cause be removed, the effect is removed.
Hence if the difference of powers came from the difference of objects, the same object would not come under different powers. This is clearly false; for the same thing is known by the cognitive power, and desired by the appetitive.
Obj. 4: Further, that which of itself is the cause of anything, is the cause thereof, wherever it is. But various objects which belong to various powers, belong also to some one power; as sound and color belong to sight and hearing, which are different powers, yet they come under the one power of common sense. Therefore the powers are not distinguished according to the difference of their objects.
_On the contrary,_ Things that are subsequent are distinguished by what precedes. But the Philosopher says (De Anima ii, 4) that "acts and operations precede the powers according to reason; and these again are preceded by their opposites," that is their objects.
Therefore the powers are distinguished according to their acts and objects.
_I answer that,_ A power as such is directed to an act. Wherefore we seek to know the nature of a power from the act to which it is directed, and consequently the nature of a power is diversified, as the nature of the act is diversified. Now the nature of an act is diversified according to the various natures of the objects. For every act is either of an active power or of a passive power. Now, the object is to the act of a passive power, as the principle and moving cause: for color is the principle of vision, inasmuch as it moves the sight. On the other hand, to the act of an active power the object is a term and end; as the object of the power of growth is perfect quantity, which is the end of growth. Now, from these two things an act receives its species, namely, from its principle, or from its end or term; for the act of heating differs from the act of cooling, in this, that the former proceeds from something hot, which is the active principle, to heat; the latter from something cold, which is the active principle, to cold. Therefore the powers are of necessity distinguished by their acts and objects.
Nevertheless, we must observe that things which are accidental do not change the species. For since to be colored is accidental to an animal, its species is not changed by a difference of color, but by a difference in that which belongs to the nature of an animal, that is to say, by a difference in the sensitive soul, which is sometimes rational, and sometimes otherwise. Hence "rational" and "irrational" are differences dividing animal, constituting its various species. In