You can fling open the doors of perception without ever taking a hallucenogenic drug -- all it takes is an understanding of how your mind works.
Your mind isn't a passive observer of reality. Much of what you “see” or otherwise sense is actually a byproduct of how your brain processes sensation.
Scientists have come up with a number of simple tricks that expose the artifice of your senses, so that you end up perceiving what you know isn't real.
