Nature of the common sense.
By means of the central sense, a knowing subject perceives the actual sensations of the external senses, distinguishes between them, and combines them.
The proper sense judges of the proper sensible by discerning it from other things which 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. Wherefore the discerning judgment must be assigned to the common sense; to which, as to a common term, all apprehensions of the senses must be referred; and by which again, all the intentions of the senses are perceived; as when someone sees that he sees. For this cannot be done by the proper sense, which only knows the form of the sensible by which it is immuted, in which immutation the action of sight is completed, and from which immutation follows another in the common sense which perceives the act of vision.(ST I, 78, 4, ad.2)
The proper object of the common sense is not the common sensibles. For common sensibles can be known by the external senses through their proper sensibles: e.g., shape can be seen through color. The proper object of the common sense is the sensations of the external senses.
Functions of the common sense.
The common sense has the following functions:(Regarding these functions, see Aristotle, De anima, III, 2 (426b8-427a15); De somno 2 (455a5). cf: St. Thomas, In de anima III, lect.3, nr. 599 ff.)
- To know all the sensations of the external senses which are known separately by the external senses.
- To compare and distinguish these qualities, e.g., color and taste.
- To be aware of the operations of the external senses.
- To distinguish the real objects from the images of the fantasy, e.g., to know whether we are dreaming, and to realize that our dreams are not reality.