The Function of the Sensus Communis
The discussion and explication of the sensus communis begins with the following passage from the Commentary on the Soul in which Aquinas considers the unique function of this internal sense faculty.
Aristotle points out the faculty that discriminates, as the particular senses cannot discriminate, between the object of the different senses; [. . .] the demonstration has three parts:
He observes that whereas we are able to distinguish not only between black and white or sweet and bitter, but also between white and sweet, and indeed between any one sense object and another, it must be in virtue of some sense that we do this; for to know sense objects as such is a mental activity of the senses. The difference between white and sweet is for us not only a difference of ideas, which would pertain to the intellect, but also precisely a difference between sense impressions, which pertains only to some sense faculty. (Commentary on the Soul, no. 601)
Aquinas explains the need for postulating the sensus communis as a specific internal sense in order to take care of mental activities that he considers as pre-analytic data. Moreover, this must be (he argues) one sense faculty and not many: ‘Aristotle shows that it is by one and the same sense that we distinguish white from sweet’ (no. 603). Aquinas has already distinguished these two proper sense objects—white and sweet— each of which is the proper object of a special sense power—sight and taste. The sensus communis conjoins these two disparate sensibles as pertaining to one collection of sensibles. Accordingly, this is not so much ‘discrimination’ as ‘conjoining’. In the following text, by considering the set of problems that would follow if sensation were exhausted in terms of the external senses alone, Aquinas postulates the need for this special internal sense faculty.
Sight does not perceive the audible as such, nor hearing the visible as such (for eye takes no impression from the audible, nor the ear from the visible), but both sensible objects are perceived by each sense only in so far as ‘one sense’, i.e. one actual sensation, so to say, bears upon an object which contains both. I mean that both the senses in question are exercised at once upon one and the same sensible thing, as when bile is at once seen as red and tasted as bitter. So that as soon as we see that this thing is red, we judge that it is bitter. But there is no external sense for the conjunction of redness and bitterness. For this conjunction is quite incidental, and what is incidental cannot be the object of any special (i.e. proper) sense faculty. (no. 581; emphasis added)
The problem Aquinas proposes is that human perceivers, to use his example, have an awareness that the same thing is both ‘red’ and ‘bitter’. This awareness of the conjunction of these two proper sensibles, however, is not a specific function or act for any external sense. It appears what Aquinas means is that this relation of conjunction of sensible qualities is not perceivable because the proper sensibles themselves are qualities and not relations. However, Aquinas claims that even though the external senses are not properly geared towards perceiving the conjunctive relations, nonetheless an object is perceived immediately as a bundle of sensations and not as a series of discrete sensibles. In order to account for this awareness of conjunction of sensible qualities, which awareness cannot be explained by means of the external senses alone, Aquinas posits the sensus communis. On this point, Aquinas and Russell, among others, would be at odds philosophically. It appears to be impossible, so Aquinas suggests, for a per- ceiver to have a direct awareness of a relation through the means of the external senses while sensing the proper sensibles. In The Problems of Philosophy, among other places, Russell argued that knowers have a direct awareness of relations. Relations were, for Russell, subsistent universal entities. Aquinas adopted an ontological position only on properties and primary substances, and not on relations. Following Aristotle, Aquinas opted for the existence of a ‘relational property’ rather than a subsisting relation as an ontological singular. This ontological position may account for Aquinas’s requirement that there be a specific faculty of internal sense, which is able to perceive directly this conjunctive relation.
This account postulating the need for the sensus communis is developed further in the Summa Theologiae. In addition, Aquinas notes that another mental activity of the sensus communis is the proper reflection of sensual awareness. It is through the sensus communis that a perceiver is aware that she indeed is sensing. In other words, this is the internal sense faculty through which a perceiver is aware that she is aware.
The proper sense judges of the proper sensible by discerning it from other sensibles that come under the same sense, for instance, by discerning white from black or green. But neither sight nor taste can discern white from sweet, because what discerns between two things must know both. Hence, the discerning judgement must be assigned to the sensus communis. To it, as to a common term, all mental acts of the external senses must be referred, and by it, again, all the intentions of the senses are perceived. For instance, when someone sees that she sees. For this cannot be done by the proper sense, which knows only the form of the sensible by which it is immuted. In this immutation, the action of sight is completed, and from it follows another immutation in the sensus communis, which perceives the act of seeing. (Summa Theologiae, I q. 78 a. 4 ad 2)
That this is one sense faculty or power is suggested in the following passage from a different part of the Summa Theologiae; this passage, referring explicitly to ‘a single sense faculty, offers additional textual evidence opposed to Phillips’s claim mentioned
above: ‘Also, it follows that in a human person, the sensus communis, which is greater
than the proper senses—although it is a single faculty—knows all the things, which are known by the five external senses’ (q. 57 a. 2).
The first role of the sensus communis is to account for the perceiver’s ability to discriminate between the different sensibles of divergent categories known by the external senses. In analysing the above passages, it becomes obvious that Aquinas’s justification for postulating the sensus communis is that the external senses themselves cannot perceive the given difference between different genera of sensibles, whether proper sensibles or common sensibles. Thus the eye might distinguish between the given ‘red’ and the given ‘blue, but it cannot distinguish the ‘c-sharp’ from the ‘blue, But Aquinas, again appealing to a pre-analytic datum of awareness, claims that human perceivers do make such distinctions. Furthermore, if such distinctions are made mentally, then a corresponding mental act of awareness is necessary in order to account for this distinction. If such a mental act exists, then there must be a corresponding ability or capacity to account for this mental act. This, in effect, is Aquinas’s argument for postulating the sensus communis as a necessary factor in completing his epistemological account of sensation. This faculty offers a naturalistic explanation of the mental acts for which it is responsible.
Although textually Aquinas places great weight on ‘discrimination’ or ‘the discerning ability’ as the chief operational characteristic of the sensus communis, it seems that this discriminating ability is treated structurally as a necessary condition for the ‘synthesizing ability’ of the sensus communis. In other words, the sensus communis is the faculty that ‘ties together’ the discrete sensible data of the external senses into concrete wholes. For example, the sensus communis brings together the red-sensible and the round-sensible with the sweet-sensible in forming the concrete sensible whole. In this case, the concrete sensible whole would be the red, round Jonathan apple here and now seen, touched, and tasted. Thus the perceiver is not limited to mere awareness of discrete psychological atoms. Rather, the perceiver, because of the function of the sensus communis, is able to be aware of concrete wholes. The bundle of sensations from the different external sense faculties is conjoined as one. From this analysis it follows that Aquinas rejects simple psychological atomism.