The Power of Reflection
In addition to discriminating between different exemplifications from the different genera of sensibles as well as unifying the concrete whole of sense experience, Aquinas claims that the sensus communis is the faculty capable of producing ‘reflexive’ mental acts in sensation. It is the internal sense faculty by which a perceiver is aware that she can be aware of seeing an object. In other words, not only is the per- ceiver seeing the object, but also she is aware that she is seeing it. It is through the mental act of the sensus communis that a perceiver is aware that she is aware. Aquinas writes that the faculty, which takes care of this mental act of reflexive awareness, is the sensus communis. It is impossible for a mental act to have any physical, sensible qualities or properties. Therefore, the act of awareness can have as its object only an existing, ‘fleshed out’ proper or common sensible. Accordingly, a mental act of seeing is not itself coloured, even though it is ‘about’—i.e. it ‘intends’—a coloured object. Aquinas’s basis for intentionality theory is brought to bear on this issue. The form of the thing known is possessed in an immaterial manner—in esse intentionale— and not in a physical, entitative manner—esse naturale.24 The result of this ‘intentional immutation’ or ‘non-material immutation’ is that the mental act in sensation lacks any physical characteristics that are existentially ‘fleshed out’ as naturally existing proper or common sensibles as found in the external world. Aquinas referring to Aristotle does use the analogy of the ring/signet in the wax example, as this often- used passage illustrates:
Aristotle finds an apt example of this in the imprint of a seal on wax. The disposition of the wax to the image is not the same as that of the iron or gold to the image. Hence, wax, he says, takes a sign, i.e. a shape or image, of what is gold or bronze but not precisely as gold or bronze. For the wax takes a likeness of the gold seal in respect of the image, but not in respect of the seal’s intrinsic disposition to be a gold seal. (Commentary on the Soul, nos 552-7)
While this is only an analogy, the point is that a ‘form’ or ‘shape’ of a thing can be transferred—‘immuted’—into another medium. It does not follow, however, that only a physiological immutation is sufficient in order to explain a human person’s awareness of a specific sense object—either a proper or a common sensible.
Aquinas affirms, however, that a perceiver is aware that she is aware; i.e. when the perceiver actually perceives (P), she can also be aware that she is perceiving (P).
To it [the sensus communis], as to a common term, all apprehensions of the senses must be referred, and by it, again, all the intentions of the senses are perceived. As 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, q. 78 a. 4 ad 2; emphasis added)
It is the sensus communis, therefore, which is the faculty of reflexive sensation. This is necessary because the external senses, insofar as they are ordered to the proper and the common sensibles, cannot perceive their own mental acts per se. This is due to Aquinas’s theory of intentionality by which a mental act itself lacks both physiologically existing proper and common sensibles. Therefore, the act of sense awareness itself cannot be an act of the external sense faculties. The sensus communis is postulated in order to take care of this pre-analytic datum of human awareness. In effect, the sensus communis becomes the ‘root’ or ‘source’ of consciousness: ‘The internal sense [called the sensus communis] is called ‘common’ not by predication, as if it were a genus, but as the common root and principle of the external senses’ (q. 78 a. 4 ad 1). ‘The object of the sensus communis is the sensible. Therefore, this includes whatever is visible and whatever is audible. It follows, then, that although the sensus communis is one power, it extends to all the objects of the five senses’ (q. 1 a. 3 ad 2). The last passage illustrates wonderfully the necessity of postulating the sensus communis in Aquinas’s philosophy of mind. This entails that the sensus communis is the source of consciousness. Sleep as well as unconsciousness are due to the non-functioning of the sensus communis2
By way of summary, there are three functions of the sensus communis:
- (a) to be directly aware of the conjunctive relation and its relata (the proper and the common sensibles) of the external senses;
- (b) to be able to have a reflective awareness of the act of awareness of the external senses;
- (c) to serve as the ‘seat of consciousness’; thus, it is through or due to the sensus communis that one is a conscious perceiver of the external world.
-  See Summa Theologiae, I q. 78 a. 3 ad 3.