In the "Phaedo", Plato makes it clear that the Forms are the "universalia ante res", i.e. they are ideal universals, by which we are able to understand the world. In his allegory of the cave, Plato likens the achievement of philosophical understanding to emerging into the sun from a dark cave, where only vague shadows of what lies beyond that prison are cast dimly upon the wall. Plato's forms are non-physical and non-mental. They exist nowhere in time or space, but neither do they exist in the mind, nor in the pleroma of matter; rather, matter is said to "participate" in form (μεθεξις, "methexis"). It remained unclear however, even to Aristotle, exactly what Plato intended by that.