The whole thing is worth reading, especially if your vision is headed for the suboptimal, but this passage has particular clarity:
All vision is in the brain, of course, and the world we see is largely a set of descriptions we project from the portfolio of things we already know. We can see how this works when we disconnect raw sensory perception from our descriptive engines. This is what happens with LSD. As I understand it (through study and not experience, alas), LSD disconnects the world we perceive from the nouns and verbs we use to describe it. So do other hallucinogens.