The Bounds of Sense
Aquinas proposes that a triadic relation only holds within certain boundaries. He often gives the example of the ill person with the feverish tongue. An object which is too intense can not only alter the sensation but also has the possibility of destroying the sense organ and consequently the sense faculty. Aquinas writes: ‘The same is clear from the fact that a sense is weakened and injured by sensible objects of extreme intensity. Thus, the ear is impaired by excessively loud sounds, and the eye by excessively bright lights. This occurs because the harmony [ratio] within the organ is shattered’ (Compendium of Theology, pt I, ch. 79). Aquinas discusses this problem of intensity:
An excess in the object destroys the sense organ. For if sensation is to take place, there must pre-exist in the organ of sense ‘a certain ratio’ or [. . . ] proportion. But the proportion is destroyed and the sense itself, which precisely consists [. . .] in the formal proportion of the organ, is neutralized. It is just as though one were to twang cords too violently, destroying the tone and harmony of the instrument, which consists in a certain proportion. (Commentary on the Soul, no. 556)
One finds similar passages, some previously noted, throughout the writings of Thomas. ‘For sense is proportioned to its organ, and in some way is assimilated to its nature. Therefore, the operation of the sense is changed even according to the change of the organ. This therefore is the meaning of the expression “not to be mixed with body”, that the intellect does not have an organ as the sense does’ (On the Unity of the Intellect Against the Averroists, ch. 1, no. 23). Commenting on this theory, Turnbull wrote that Aristotle’s account ‘is also, of course, like Plato’s, one in which the active powers of the thing and the passive powers of the sensitive organ are both needed for the perception to occur’. Turnbull notes that the ‘ratio’ is a kind of ‘mean’, beyond which the sense organ cannot function in its proper mode.
-  Robert G. Turnbull, ‘The Role of the “Special Sensibles” in the Perception Theories of Plato andAristotle, in Robert G. Turnbull and Peter Machamer (eds), Studies in Perception (Columbus: Ohio StateUniversity Press, 1978), 23.