Comment by torginus

Aren't these the same thing, to some extent? I remember my music teacher told me, that as long as you can hear your false tones, she can teach you how to sing, no matter how bad you are at it. But if you can't hear it, she cannot help you.

AI is the same - as long as it can see well, it can tell the difference between what it outputs and what its supposed to. If you subtract the two, you have an error, and you can hill-climb on that.