Full Idea
If the eye were an animal, sight would have been its soul, for sight is the substance or essence of the eye which corresponds to the formula, the eye being merely the matter of seeing; when seeing is removed it is no longer an eye,except in name.
Gist of Idea
Sight is the essence of the eye, fitting its definition; the eye itself is just the matter
Source
Aristotle (De Anima [c.329 BCE], 412b18)
Book Reference
Aristotle: 'The Basic Works of Aristotle', ed/tr. McKeon,Richard [Modern Library Classics 2001], p.556
A Reaction
This is a drastic view of form as merely function, which occasionally appears in Aristotle. To say a blind eye is not an eye is a tricky move in metaphysics. So what is it? In some sense it is still an eye.
Related Idea
Idea 16753 Giving the function of a house defines its actuality [Aristotle]