Continuing his account of the senses, Aristotle asserts that there is no sense other than the five: sight, hearing, smell, taste, and touch. But how do the various senses perceive objects that are common to more than one sense? There must be a larger unity to sensation, above and beyond the individual senses. This unity is provided by the fact that the elements act as mediums for perception. Since there are four elements—air, fire, earth, and water—which act as mediums of sense perception, it follows that all sense organs are composed of these elements since the sense organ must be akin to its medium.

Having completed his account of the senses, Aristotle now seeks to answer some new questions. How do we explain the fact that we are aware of our sense perceptions—that we in a sense see ourselves seeing? Is there then an additional sense? Do the senses perceive themselves? Secondly, how do we distinguish one sense from another? How do we know that we are, say, seeing a black color rather than tasting a sweet flavor?

Aristotle concludes that there must be a larger principle of unity in the senses that explains how they interact.