Three problems with backpropagation
	• Where does the supervision come from? – Most data is unlabelled • The vestibular-ocular reflex is an exception. • How well does the learning time scale? – Its is impossible to learn features for different parts of an image independently if they all use the same error signal. • Can neurons implement backpropagation? – Not in the obvious way. •  but getting derivatives from later layers is so important that evolution may have found a way.