Surrealism can, when well executed, challenge what the viewer perceives as reality. Often this challenge comes in the form of optical illusions.
In this case Rene Magritte has clearly drawn a pipe, then tagged it with the line “This is not a pipe.”
Despite what our eyes see, it is not a pipe. It is merely a drawing of a pipe. The illusion here is not so much visual as perceptive.
What else in the world do we perceive that is not truly what it seems to be?