The Necker Cube is an optical illusion and an ambiguous line drawing. The effect is interesting because each part of the picture is ambiguous by itself, yet the human visual system picks an interpretation of each part that makes the whole consistent.
The Necker Cube is used in epistemology and provides a counter-attack against Naïve Realism(common-sense realism) that states the way humans perceive the world is the way the world actually is. The Necker Cube seems to disprove this claim because we see one or the other of two cubes, but really, there is no cube there at all: only a two-dimensional drawing of twelve lines. We see something which is not really there, thus disproving naïve realism.