Memory can come from direct perception, false knowledge, verbal delusion, and sleep. For instance, you hear a word. That word is like a stone thrown into the lake of the Chitta; it causes a ripple, and that ripple rouses a series of ripples; this is memory. So in sleep. When the peculiar kind of ripple called sleep throws the Chitta into a ripple of memory, it is called a dream. Dream is another form of the ripple which in the waking state is called memory.