All things are hard:
man cannot explain them by word.
The eye is not filled with seeing,
neither is the ear filled with hearing (Eccl. 1:8)
Sensation, on the part of its object, is the most known power of the soul; since all knowledge has its origin in the imagination which in turn is derived from sense. Yet, considered as a power of the soul, it offers many puzzles. In solving a puzzle, one begins with what is most known and deduces what is less known from it. Thus, in discovering how sensation is like motion, it is necessary to consider motion, and then see how the parts of motion correspond to the parts of sensation.
The first thing one notices about motion, is that there is a thing moved and something which moves it. Considering this, Aristotle proposes a definition of motion: it is the actuality of what exists in potency as such2 Thus we see that every motion involves an actuality and a potency. We also see that the actuality in motion is imperfect, since while it occurs, it is in potency in some respect. Finally, we see that the actuality is in what is potential, since that is what comes into actuality. In sensation there is both a mover and a moved, but which part is called sensitive and which sensible is ambiguous. In addition to this puzzle, if we are to see how it is similar to and different from motion, it will be necessary to show how it involves act and potency, whether the act involved is imperfect, and where the actuality is. Let us go through the difficulties to show that sensation is composed of two motions: one proceeding from the sensible on the sensitive, and one which begins and ends in the sensitive insofar as it has been perfected through the reception of a form.
Perhaps the first notion of what is actual and what is potential is found in the names given to the principles of sensation: sensitive, and sensible. From these names, it seems that the sensitive is the agent, and the sensible the potential. If we should consider them this way, we would say that sensation is the actuality of the sensible as sensible. From this we would conclude that the sense is the agent, and the sensible what exists in potency, and that the actuality is in the sensible.
But if this were the case, it would seem that the sense itself would be sensed. For, the sensitive soul would be complete with respect to its act. That is, just as the locomotive soul, exterior impediments aside, can move the body whenever it wishes, so also the sensitive soul should be able to sense whenever it wishes; for organs of sense are themselves sensible. But it is manifest that we do not sense the organs by which we sense, but something exterior. Therefore the sensitive soul cannot be complete in itself regarding its own act.
From the preceding we can see that the sensitive is a power of the animal which requires something external to complete it. Thus, the sense-power is seen to be a certain potency to the sensible form. Considered this way, the potency is in the sensitive, the actuality in the sensible, and the sensible moves the sensitive. From this it follows that the actuality of the sensible is in the sensitive.
However, in order to understand this, potency must be understood as having a twofold signification. Thus, in one way it signifies the ability to receive a form as if it were a quality in a subject. Visible bodies have a potency to colored in this way. For example, the green apple has a potency to red insofar as it can become a red apple. It is clear, then, that the sense cannot be potential in this way. For if it were, the organ would receive the sensible form as a quality, and the color could not be sensed since that which is applied to the organ is not sensed. Thus, the sense must be potential in a different way.
The other way something is potential, is called potency from its receptivity3, and this is the way in which the sense is potential. Thus, as Aristotle points out4, sense is a power which receives the form without the matter. This is similar to how a wax tablet can receive the shape of a seal which is impressed on it without becoming that seal. So also, some things are receptive of sensible qualities without posessing them as the matter. One such is the transparent; for if the transparent were to receive the coloreds as qualities, then the space around the visible object would be colored. And since colored between the eye and the colored object obscures the colored object, the sensation would not be of the thing but of the transparent. Further, in these conditions sight would not happen since the color would be placed on the eye5 Sense, then, is potential insofar as it receives the sensible without becoming sensible itself. Thus, we can still say that the sensible acts on the sense, that its actuality is in the sense, and that it alters the sense.
But now we have two related difficulties: first, if sense and the transparent both receive the sensible form in the same way, the effect produced should be the same; second, if the sense is passive as was just shown, it should be called “sensible” rather than “sensitive,” which would go against the common opinion expressed in speech that sensation is an action which comes from the soul as an agent. Aristotle also notices this at the very end of book two of theDe Anima where he notes:
What, therefore, is smelling except suffering something? Or is not smelling also sensing, while the air, suffering quickly, becomes sensible?6
Therefore the suffering that occurs in the sense is different from that which occurs in the medium insofar as both suffer, yet only the sense acts. Aristotle gives us a clue as to how this is when he notes that motion is an imperfect act while sensation is the act of the perfected7. If sense, then, is some action, it would seem odd to call it a passive power. Yet, this is how we speak of it and is implied in calling it the act of the perfected. Let us untie this knot and discover how sense is passive, and how active.
To begin the untying it is necessary to meditate on how Aristotle describes this receptivity. He calls it the “saving of a being in potency by a being in actuality8.” When Saint Thomas explains this passage, he calls it a perfection9. From this the distinction between active and passive powers is seen: active powers, when in potency, are sufficient in such a way that they can be used at will; passive powers, on the other hand, must be altered by something outside, before they attain this level of sufficiency, when they have been altered, their act happens simultaneously with the end of the alteration. Thus, where motion is the act of the imperfect as such, sensation is the act of the sense-power when it has been perfected. That is to say that sensation is the act of a power which cannot operate at will, but only when it has been perfected through something altering it.
Thus we see that sensation is like motion, suffering, and alteration in two ways: first, it is like them insofar as the sense is brought from potency to act by the agency of the sensible, in this way it is altered, undergoes, and is moved as has been said; secondly, it is like them insofar as it makes the sensible to be sensed, and in this way it alters and moves the sensible. Yet, this second motion is not motion univocally, but it is motion in a certain way.
1 Ecclesiastes 1:8
2 On Natural Hearing Book III, 201a11
3 St. Thomas Setentiis de Anima, Bk. II Lectio XI, n.9
4 De Anima 424b16
5 De Anima 419a13; I have a theory about why the medium is necessary, but I’m not sure how Aristotle considers it. It seems to me that the medium is necessary, because it is the instrument by which color can become immaterial.
6 Aristotle, De Anima 414b16
7 De Anima, 431a6
8 De Anima 417b3
9 De Anima 417b4; St. Thomas says this in his commentary on the passage (Sententiis De Anima III, lectio 11, n.8): Actus autem est perfectio potentiae; et ideo hoc modo dicitur passio, non secundum quod fit quaedam corruptio patientis, sed magis secundum quod fit quaedam salus et perfectio eius quod est in potentia, ab eo quod est in actu.
10 De Anima 431a7