The Sensus Communis and the External Sensorium
In order to resolve this quandary over historical interpretations of Aquinas on perception, one must recall the distinction between ‘the external sense faculties' and ‘the external sensorium’. All along, Aquinas includes the sensus communis among his list of internal senses. ‘So there is no need to assign more than four internal powers of the sensitive part—namely, the sensus communis, the imagination, and the aestimativa (or the vis cogitativa in human beings) and the memorativa powers’ (Summa Theologiae,
Ia q. 87 a. 4). In De Veritate, Aquinas writes: ‘the organs of the power of imagination, of memory, and of the vis cogitativa, are in the brain itself, which is the place of greatest moistness in the human body.’
Aquinas places the sensus communis with the internal sense faculties because the physiology he adopted, most probably from the Arabian philosophers, asserts that the bodily organ or vehicle where the sensus communis is found is located somewhere in the brain. With this physiological location, it could not be classified an external sense organ. Structurally, however, the sensus communis is part of the external sensorium because its object is the collection of proper and common sensibles that are the sensible objects of the external senses. Its object is not an image, phantasm, or any type of post-sensation mental entity. The object of the sensus communis is a concrete whole unified from the discrete data—the assorted collection of proper and common sensibles of the external senses.
The division between internal and external sensoria, on the other hand, is determined by means of the function of the corresponding mental acts. The function of the external sensorium is to be aware of the sensible qualities in the external world— rooted in the primary substances of natural kinds—which are present immediately to the perceiver in a causally efficacious way. These, of course, are the proper and common sensibles. If there were no sensible objects, then there would be no mental acts with the external sensorium. The internal sensorium, on the other hand, has the ability both to ‘remember’ what has been perceived and to ‘interpret’ what is presently experienced. Both these functions go beyond the immediate data of the external sensorium. What distinguishes the internal from the external sensorium is the presence of phantasms. In other words, the role of phantasms is a necessary condition for inner sense in Aquinas’s philosophy of mind. However, a phantasm is never connected only with the external sensorium. Kemp’s analysis entails placing the sensus communis with the internal sensorium. If this were the case, then it follows that images or phantasms are necessary conditions for the sensus communis. If the sensus communis did have a phantasm as the direct object of its mental act, it would follow that Aquinas is a rep- resentationalist. It appears that those historians of philosophy who suggest that a phantasm is the direct object of the sensus communis fail to reconcile the ramifications of this consequent representationalism with Aquinas’s strong assertion of direct realism and externalism. Furthermore, there do not appear to be any texts in which Aquinas places a phantasm with a mental act of the sensus communis.
These two categories of sense and sensorium, noted earlier, are neither equivalent nor coextensive. In other words, the external senses are not to be equated with the external sensorium; neither are the internal senses to be equated with the internal sensorium; the difference consists in the conceptual analysis of the sensus communis. The
sensus communis, although an internal sense, is part of the external sensorium and not part of the internal sensorium. This claim separating the sensus communis from the internal sensorium depends upon the placement of a phantasm. The internal sensorium makes use of a phantasm in all three of its faculties. Insofar as the sensus communis lacks a phantasm, then it cannot be a part of the internal sensorium or inner sense. To conclude, the external sensorium has for objects of its act of awareness the various proper and common sensibles. The internal sensorium, by means of phantasms, has retentive and interpretive functions to perform. These mental acts of inner sense are distinct structurally from the external sensorium. The phantasm, therefore, is the critical epistemological entity grounding the distinction between the external and the internal sensorium.
-  Aquinas notes that Avicenna held that there were five internal sense faculties.
-  In his lecture notes on Aquinas and inner sense, Michael Stock rendered this distinction betweensense and sensorium: Psychologia (Dover, Mass.: St Stephen’s College, 1960). Stock’s work is one of the fewAquinas studies where this distinction is discussed.
-  Summa Theologiae, I q. 78 a. 4. In article 4, ‘Utrum Interferes Sensus Convenienter Distinguantur, thesensus communis is listed as one of the internal senses.