The Directly Perceivable and the Indirectly Perceivable
In considering the objects of sensation, it was noted above that Aquinas offers two generic types of such objects.
- (a) the objects that are directly perceived, which are the proper and the common sensibles;
- (b) the objects that are accidentally perceived, which are the incidental objects of sense.
In the Supplement to the Summa Theologiae, we find a discussion about the distinction between the ‘directly perceptible’ and the ‘indirectly perceptible’ articulated in some detail.11 Of course, the concepts of ‘directly perceivable and ‘indirectly perceivable’ require explication and analysis. At first glance, one might suggest that Aquinas uses this distinction between the directly perceivable and the indirectly perceivable in order to refer to two types of awareness. One kind would be an awareness of an object known only by means of an inference.
At this point, moreover, another distinction is necessary in sorting out Aquinas’s account of sense knowledge. So far the terms ‘sensation’ and ‘perception’ have been used almost interchangeably. Textually, in fact, Aquinas seems not to propose a distinction either between these two terms or between the respective mental acts to which they refer and their respective objects. By applying the above terminology—the directly perceivable and the indirectly perceivable—a distinction between sensation and perception can be offered. In Aristotle and Aquinas, the directly perceivable objects are the proper sensibles and the common sensibles. The indirectly perceivable objects, on the other hand, are the incidental objects of sensation. Aquinas considers both these categories to be mental acts of sensation because neither makes use of first or second intentions at the level of concept formation or the awareness of universals. The indirectly sensible, while not an object of the external sensorium, is still not the object of a conceptus of the intellect. Rather, this category of sensibles is known through one of the mental acts of one of the faculties of the internal sensorium. The mental acts of the external senses are necessary but not sufficient conditions for the mental acts of the internal sensorium. This is the basis for Aquinas’s distinction between the direct 
and indirect object of sensation, and for his distinction between sensation and perception.
In summary form, therefore, ‘sensation’ refers to the workings of the external senso- rium. ‘Perception’ refers to the workings of the vis cogitativa, which has for its object the incidental object of sense, the Aristotelian primary substance; these are the particular instances of substantial things that exist in Aquinas’s world. Nonetheless, it is through the distinction between the external senses and the awareness by the vis cogitativa that Aquinas can affirm the distinction between sensation and perception. This is also his way of offering a more nuanced analysis than Thomas Reid provides for the same set of issues.
-  This text occurs later in this chapter (Summa Theologiae, Supp. to III, q. 92 a. 2).