We do not perceive what is "out there," rather we perceive what is "in here." Our senses can only inform us of their own status. They can inform us of the electrical status of neurons or the physical or the chemical status of the receptors. The outside world is never taken into our consciousness. The outside world is rather our own creation, psychologically synthesized from the mass of sensations that envelope us. In many respects, the ultimate question that perception must ask was stated by John Stuart Mill in 1865. He asked, "What is it we mean, or what is it which leads us to say, that the objects we perceive are external to us, and not a part of our own thoughts?" That remains, perhaps, the ultimate, unresolved perceptual puzzle.

Stanley Coren, Sensation and Perception