ILLUSION TYPICALLY REFERS TO A SENSORY—USUALLY visual—misperception. Thousands of optical illusions are available, with new ones continually devised or discovered. In the context of such perceptual illusions, the word isn’t used disparagingly. On the contrary, everyone with normal vision sees them, and they’re a continual source of puzzlement and delight. They’re also of scientific interest for what they reveal about visual perception. Optical illusions, in fact, are so ubiquitous that we forget that our modern movie, television, computer, and smartphone screens all depend on them: nothing we see on our screens is actually there. The impression that anything is moving, too, is an illusion created by the rapid display of a succession of still images.
In a broader sense, though, illusion refers not to a sensory misperception but an intellectual one (especially one due to deception or self-deception). When someone is using illusion in this sense, the phrase nothing but is implicit—like Alice crying “You’re nothing but a pack of cards!” In this broader sense, illusion is nearly synonymous with delusion. The original, broader sense is evident in such words as disillusion and disillusionment. In science, few concepts have been more often targets of attack as illusion than time, causality, consciousness, the self, and free will. I will make the case that they’re not illusions at all.