The Sensus Communis as the Root of Sensation
Aquinas claims that the sensus communis is the structural root or source for the workings of the external sensorium. He appears to build on this ‘common root’ idea by considering what must be necessary if discrimination of sensibles is to occur. In order to explain this argument, we must consider several texts from the Commentary:
Aristotle shows that it is by one and the same sense that we distinguish white from sweet. For one might have supposed that we did it by different senses, by tasting sweetness and by seeing whiteness. But if this were true, he says, we could never perceive that white was other than sweet. If this difference is to appear [i.e. to be perceived], it must appear to some one sense faculty. So long as white and sweet are sensed by distinct faculties, it is as though they were sensed by two different human perceivers, one perceiving sweet and another white: I this and you that. In this case, ‘a’ (sweet and white) and ‘b’ (white) are obviously distinct, because I am impressed in one way by sweetness and you in another by whiteness. (Commentary on the Soul, no. 603)
Aquinas proceeds to elaborate on this distinction:
But this alone would not show us their sensible difference. There must be one single faculty, which ‘says’ that sweet is not white, precisely because this distinction is one single object of knowledge. The ‘saying’ is the expression of an inward knowing; and as the saying is a single act, it must spring from a single act of understanding and sensing that what is sweet is not white. [. . .] As when the human person who judges white to be other than sweet, must be one knower who is aware of both objects, so she must do this by means of one faculty. For awareness is the act of a cognitive faculty. Hence, Aristotle’s conclusion, that it is clearly impossible to perceive ‘separate’, i.e. by distinct, means; there must be one single power aware of both sensible objects. (no. 604)
Aquinas continues his argument in establishing the unity of the sensus communis.
The judgement of difference is in the present in the sense that there is a difference at present; which necessarily implies a simultaneous apprehension of the two different objects. They are both known in the same instant as they are known to be different. Obviously, then, they are known at once and together. Hence, as one undivided faculty perceives the object’s difference, so in one undivided moment both are apprehended. (no. 605)
Using an analogy of a point and a line, he argues for the unity of perception:
Aristotle gives the correct solution [to the sensus communis], using the simile of a point. Any point between the two ends of a line can be regarded either ‘as one or two’. It is one as continuing the parts of the line that lie on either side of it, and so forming the term common to both. It is two inasmuch as we use it twice over, to terminate one part and begin the other. (no. 609)
Aquinas next applies this analogy to the sensus communis:
Now sensitivity flows to the organs of all the five senses from one common root, to which in turn are transmitted, and in which are terminated, all the sensations occurring in each particular external sense organ. And this common root can be regarded from two points of view:
Hence, what Aristotle means is that, just as a point, under a certain aspect, is not one only but also two or divisible, so the principle of sensitivity, if regarded as the root and term of seeing and of hearing, appears twice over under the same name, and in this way it is divisible. (no. 169)
In this schema, the sensus communis accounts for the unity of the external sensorium: ‘The sensus communis is a common sensitive principle, aware of several objects at once because it terminates several organically distinct sensations; and as such, its functions are separate. It is because it is one in itself that it discerns the difference between these sensations’ (no. 610).
To put the matter a bit differently, insofar as each external sense faculty is defined only in terms of one generic type of proper sensible, there cannot be a cross-reference by using the activities of the external sense faculties alone. As a consequence of this impossibility of faculty cross-reference among the external senses themselves, Aquinas argues for the need for an additional faculty, which can explain this cross-reference. He is convinced that this cross-reference occurs as a pre-analytic datum of experience. The required faculty to explain human awareness of this pre-analytic datum is the sensus communis. The function of the sensus communis, consequently, is not only one of discrimination. Rather, this discrimination is a necessary condition for unifying the discrete data perceived by the external senses. Therefore, insofar as Aquinas is convinced that perceivers do experience ‘concrete wholes’ and not mere ‘psychological atoms’ or discrete ‘sense data’, he postulates the sensus communis. This faculty explains the perceiver’s awareness of sensible wholes, even though the immediate data of the external senses are solely in terms of discrete sensibles—the individual proper and the common sensibles. Accordingly, the sensus communis accounts for the unity of sensation in the external sensorium.