The Necker cube illustrates how the eye can reach two conclusions. Each of the two squares in the Necker cube can be perceived as either the front or the rear surface of the cube. Try to make the cube switch back and forth between these two orientations. Now try to hold one orientation. Most people cannot maintain the whole cube in consciousness for longer than about three seconds it seems to flip from one orientation to another.
The cube appears to flip between orientations because the brain develops two equally plausible hypotheses and is unable to decide between them.
The Necker Cube protects against common sense realism, which states that the way we perceive the world is the way the world actually is.
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