This chapter is focused on Aristotle’s account of sensibility in De Anima II 12. My thesis is that the account defines sensibility as the standard in relation to which perceptible qualities are the sorts of quality they are. To illustrate, Aristotle holds that some colors are dark, others light; this implies that the spectrum of dark and light is “divided” into two “sides,” one dark, the other light. In the previous chapter I suggested that what it is for a quality to lie on one side of a spectrum – e.g. to be one of the dark colors – is a matter of its relation to the “middle” of the associated spectrum. In this chapter I argue that the claim that sensibility just is “as it were a kind of mean of the contrariety in perceptible qualities” implies that these “middles” are defined by sensibility itself, the form or essence of the primary sense organ. The upshot is that the senses are “forms” or “standards” of perceptible qualities, in that the particular qualities known by their means are the sorts of quality they are (e.g. dark colors or light ones) thanks to their relationship to the form of the primary sense organ.