The purpose of an eye is to see, but it cannot see when parted from its body.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“for the present, I will only observe that, in Aristotle’s system, the soul is what makes the body one thing, having unity of purpose, and the characteristics that we associate with the word “organism.” The purpose of an eye is to see, but it cannot see when parted from its body. In fact, it is the soul that sees. It would seem, then, that “form” is what gives unity to a portion of matter, and that this unity is usually, if not always, teleological. But “form” turns out to be much more than this, and the more is very difficult.” source