Summa Theologica, Part I (Prima Pars) From the Complete...

Reply Obj. 4: Although sense and appetite are principles of movement in perfect animals, yet sense and appetite, as such, are not sufficient to cause movement, unless another power be added to them; for immovable animals have sense and appetite, and yet they have not the power of motion. Now this motive power is not only in the appetite and sense as commanding the movement, but also in the parts of the body, to make them obey the appetite of the soul which moves them. Of this we have a sign in the fact that when the members are deprived of their natural disposition, they do not move in obedience to the appetite. _______________________

Objection 1: It would seem that the parts of the vegetative soul are not fittingly described—namely, the nutritive, augmentative, and generative. For these are called "natural" forces. But the powers of the soul are above the natural forces. Therefore we should not class the above forces as powers of the soul.

Obj. 2: Further, we should not assign a particular power of the soul to that which is common to living and non-living things. But generation is common to all things that can be generated and corrupted, whether living or not living. Therefore the generative force should not be classed as a power of the soul.

Obj. 3: Further, the soul is more powerful than the body. But the body by the same force gives species and quantity; much more, therefore, does the soul. Therefore the augmentative power of the soul is not distinct from the generative power.

Obj. 4: Further, everything is preserved in being by that whereby it exists. But the generative power is that whereby a living thing exists. Therefore by the same power the living thing is preserved. Now the nutritive force is directed to the preservation of the living thing (De Anima ii, 4), being "a power which is capable of preserving whatever receives it." Therefore we should not distinguish the nutritive power from the generative.

I answer that, The vegetative part has three powers. For the vegetative part, as we have said (A. 1), has for its object the body itself, living by the soul; for which body a triple operation of the soul is required. One is whereby it acquires existence, and to this is directed the generative power. Another is whereby the living body acquires its due quantity; to this is directed the augmentative power. Another is whereby the body of a living thing is preserved in its existence and in its due quantity; to this is directed the nutritive power.

We must, however, observe a difference among these powers. The nutritive and the augmentative have their effect where they exist, since the body itself united to the soul grows and is preserved by the augmentative and nutritive powers which exist in one and the same soul. But the generative power has its effect, not in one and the same body but in another; for a thing cannot generate itself. Therefore the generative power, in a way, approaches to the dignity of the sensitive soul, which has an operation extending to extrinsic things, although in a more excellent and more universal manner; for that which is highest in an inferior nature approaches to that which is lowest in the higher nature, as is made clear by Dionysius (Div. Nom. vii). Therefore, of these three powers, the generative has the greater finality, nobility, and perfection, as the Philosopher says (De Anima ii, 4), for it belongs to a thing which is already perfect to "produce another like unto itself." And the generative power is served by the augmentative and nutritive powers; and the augmentative power by the nutritive.

Reply Obj. 1: Such forces are called natural, both because they produce an effect like that of nature, which also gives existence, quantity and preservation (although the above forces accomplish these things in a more perfect way); and because those forces perform their actions instrumentally, through the active and passive qualities, which are the principles of natural actions.

Reply Obj. 2: Generation of inanimate things is entirely from an extrinsic source; whereas the generation of living things is in a higher way, through something in the living thing itself, which is the semen containing the principle productive of the body. Therefore there must be in the living thing a power that prepares this semen; and this is the generative power.

Reply Obj. 3: Since the generation of living things is from a semen, it is necessary that in the beginning an animal of small size be generated. For this reason it must have a power in the soul, whereby it is brought to its appropriate size. But the inanimate body is generated from determinate matter by an extrinsic agent; therefore it receives at once its nature and its quantity, according to the condition of the matter.

Reply Obj. 4: As we have said above (A. 1), the operation of the vegetative principle is performed by means of heat, the property of which is to consume humidity. Therefore, in order to restore the humidity thus lost, the nutritive power is required, whereby the food is changed into the substance of the body. This is also necessary for the action of the augmentative and generative powers. _______________________

Objection 1: It would seem inaccurate to distinguish five exterior senses. For sense can know accidents. But there are many kinds of accidents. Therefore, as powers are distinguished by their objects, it seems that the senses are multiplied according to the number of the kinds of accidents.

Obj. 2: Further, magnitude and shape, and other things which are called "common sensibles," are "not sensibles by accident," but are contradistinguished from them by the Philosopher (De Anima ii, 6). Now the diversity of objects, as such, diversifies the powers. Since, therefore, magnitude and shape are further from color than sound is, it seems that there is much more need for another sensitive power than can grasp magnitude or shape than for that which grasps color or sound.

Obj. 3: Further, one sense regards one contrariety; as sight regards white and black. But the sense of touch grasps several contraries; such as hot or cold, damp or dry, and suchlike. Therefore it is not a single sense but several. Therefore there are more than five senses.

I answer that, The reason of the distinction and number of the senses has been assigned by some to the organs in which one or other of the elements preponderate, as water, air, or the like. By others it has been assigned to the medium, which is either in conjunction or extrinsic and is either water or air, or such like. Others have ascribed it to the various natures of the sensible qualities, according as such quality belongs to a simple body or results from complexity. But none of these explanations is apt. For the powers are not for the organs, but the organs for the powers; wherefore there are not various powers for the reason that there are various organs; on the contrary, for this has nature provided a variety of organs, that they might be adapted to various powers. In the same way nature provided various mediums for the various senses, according to the convenience of the acts of the powers. And to be cognizant of the natures of sensible qualities does not pertain to the senses, but to the intellect.

The reason of the number and distinction of the exterior senses must therefore be ascribed to that which belongs to the senses properly and per se. Now, sense is a passive power, and is naturally immuted by the exterior sensible. Wherefore the exterior cause of such immutation is what is per se perceived by the sense, and according to the diversity of that exterior cause are the sensitive powers diversified.

Now, immutation is of two kinds, one natural, the other spiritual. Natural immutation takes place by the form of the immuter being received according to its natural existence, into the thing immuted, as heat is received into the thing heated. Whereas spiritual immutation takes place by the form of the immuter being received, according to a spiritual mode of existence, into the thing immuted, as the form of color is received into the pupil which does not thereby become colored. Now, for the operation of the senses, a spiritual immutation is required, whereby an intention of the sensible form is effected in the sensile organ. Otherwise, if a natural immutation alone sufficed for the sense's action, all natural bodies would feel when they undergo alteration.

But in some senses we find spiritual immutation only, as in sight: while in others we find not only spiritual but also a natural immutation; either on the part of the object only, or likewise on the part of the organ. On the part of the object we find natural immutation, as to place, in sound which is the object of hearing; for sound is caused by percussion and commotion of air: and we find natural immutation by alteration, in odor which is the object of smelling; for in order to exhale an odor, a body must be in a measure affected by heat. On the part of an organ, natural immutation takes place in touch and taste; for the hand that touches something hot becomes hot, while the tongue is moistened by the humidity of the flavored morsel. But the organs of smelling and hearing are not affected in their respective operations by any natural immutation unless indirectly.

Now, the sight, which is without natural immutation either in its organ or in its object, is the most spiritual, the most perfect, and the most universal of all the senses. After this comes the hearing and then the smell, which require a natural immutation on the part of the object; while local motion is more perfect than, and naturally prior to, the motion of alteration, as the Philosopher proves (Phys. viii, 7). Touch and taste are the most material of all: of the distinction of which we shall speak later on (ad 3, 4). Hence it is that the three other senses are not exercised through a medium united to them, to obviate any natural immutation in their organ; as happens as regards these two senses.

Reply Obj. 1: Not every accident has in itself a power of immutation but only qualities of the third species, which are the principles of alteration: therefore only suchlike qualities are the objects of the senses; because "the senses are affected by the same things whereby inanimate bodies are affected," as stated in Phys. vii, 2.

Reply Obj. 2: Size, shape, and the like, which are called "common sensibles," are midway between "accidental sensibles" and "proper sensibles," which are the objects of the senses. For the proper sensibles first, and of their very nature, affect the senses; since they are qualities that cause alteration. But the common sensibles are all reducible to quantity. As to size and number, it is clear that they are species of quantity. Shape is a quality about quantity, since the notion of shape consists of fixing the bounds of magnitude. Movement and rest are sensed according as the subject is affected in one or more ways in the magnitude of the subject or of its local distance, as in the movement of growth or of locomotion, or again, according as it is affected in some sensible qualities, as in the movement of alteration; and thus to sense movement and rest is, in a way, to sense one thing and many. Now quantity is the proximate subject of the qualities that cause alteration, as surface is of color. Therefore the common sensibles do not move the senses first and of their own nature, but by reason of the sensible quality; as the surface by reason of color. Yet they are not accidental sensibles, for they produce a certain variety in the immutation of the senses. For sense is immuted differently by a large and by a small surface: since whiteness itself is said to be great or small, and therefore it is divided according to its proper subject.

Reply Obj. 3: As the Philosopher seems to say (De Anima ii, 11), the sense of touch is generically one, but is divided into several specific senses, and for this reason it extends to various contrarieties; which senses, however, are not separate from one another in their organ, but are spread throughout the whole body, so that their distinction is not evident. But taste, which perceives the sweet and the bitter, accompanies touch in the tongue, but not in the whole body; so it is easily distinguished from touch. We might also say that all those contrarieties agree, each in some proximate genus, and all in a common genus, which is the common and formal object of touch. Such common genus is, however, unnamed, just as the proximate genus of hot and cold is unnamed.

Reply Obj. 4: The sense of taste, according to a saying of the Philosopher (De Anima ii, 9), is a kind of touch existing in the tongue only. It is not distinct from touch in general, but only from the species of touch distributed in the body. But if touch is one sense only, on account of the common formality of its object: we must say that taste is distinguished from touch by reason of a different formality of immutation. For touch involves a natural, and not only a spiritual, immutation in its organ, by reason of the quality which is its proper object. But the organ of taste is not necessarily immuted by a natural immutation by reason of the quality which is its proper object, so that the tongue itself becomes sweet and bitter: but by reason of a quality which is a preamble to, and on which is based, the flavor, which quality is moisture, the object of touch. _______________________

Objection 1: It would seem that the interior senses are not suitably distinguished. For the common is not divided against the proper. Therefore the common sense should not be numbered among the interior sensitive powers, in addition to the proper exterior senses.

Obj. 2: Further, there is no need to assign an interior power of apprehension when the proper and exterior sense suffices. But the proper and exterior senses suffice for us to judge of sensible things; for each sense judges of its proper object. In like manner they seem to suffice for the perception of their own actions; for since the action of the sense is, in a way, between the power and its object, it seems that sight must be much more able to perceive its own vision, as being nearer to it, than the color; and in like manner with the other senses. Therefore for this there is no need to assign an interior power, called the common sense.

Obj. 3: Further, according to the Philosopher (De Memor. et Remin. i), the imagination and the memory are passions of the "first sensitive." But passion is not divided against its subject. Therefore memory and imagination should not be assigned as powers distinct from the senses.

Obj. 4: Further, the intellect depends on the senses less than any power of the sensitive part. But the intellect knows nothing but what it receives from the senses; whence we read (Poster. i, 8), that "those who lack one sense lack one kind of knowledge." Therefore much less should we assign to the sensitive part a power, which they call the "estimative" power, for the perception of intentions which the sense does not perceive.

Obj. 5: Further, the action of the cogitative power, which consists in comparing, adding and dividing, and the action of the reminiscence, which consists in the use of a kind of syllogism for the sake of inquiry, is not less distant from the actions of the estimative and memorative powers, than the action of the estimative is from the action of the imagination. Therefore either we must add the cognitive and reminiscitive to the estimative and memorative powers, or the estimative and memorative powers should not be made distinct from the imagination.

Obj. 6: Further, Augustine (Gen. ad lit. xii, 6, 7, 24) describes three kinds of vision; namely, corporeal, which is the action of the sense; spiritual, which is an action of the imagination or phantasy; and intellectual, which is an action of the intellect. Therefore there is no interior power between the sense and intellect, besides the imagination.

I answer that, As nature does not fail in necessary things, there must needs be as many actions of the sensitive soul as may suffice for the life of a perfect animal. If any of these actions cannot be reduced to the same one principle, they must be assigned to diverse powers; since a power of the soul is nothing else than the proximate principle of the soul's operation.