It’s an interesting philosophical/psychological question whether it’s even possible for the human brain to be objective about anything. There is a disconnect between perception and reality—our brain is an imperfect interface to the world around us, doing its best to interpret the signals it receives, and occasionally totally falling down on the job.
I’m sure you’ve seen these great mind-bending optical illusions, like the spinning dancer above (which way is she really spinning? I see her spinning counter-clockwise, but my wife and kids see her spinning clockwise), or the Ebbinghaus Illusion at left (yes, the orange balls are actually the same size).
But have you heard any good auditory illusions lately? Try these: