Imagine a penny. If you're like most people, you know just enough about a penny to distinguish it from all the other coins. "Ah yes, the red one!" If you know any of the following, consider yourself part of a rare breed:
- Which direction Lincoln is facing
- Where the year is stamped
- Where the minting facility is stamped
- How many times total (both faces of the coin) the words are "One Cent" stamped
These are stimuli that occur to us but which we filter out because they are unnecessary information: while your perceive everything, you only regard the minimum necessary to satisfy a situation. Because you never need to know these details of the penny, you find yourself unable to remember them right now, even though you see pennies all the time. Or if you don't, imagine something else that you do see all the time. Money & credit cards, plant life around your home, or the architecture of buildings along your commute but which aren't ever your destination are all common examples. It is this ability to filter that allows us to navigate complicated city routes without knowing the street names: we don't know what the front of every building on every street in the city looks like, but we know just enough to navigate using a few landmarks. Our brain has boiled down the entire town to just a small handful of landmarks for a given trip and in the process saved much time, certainly an evolutionary advantage.
The marvelous perception-altering nature of LSD is such that you perceive the wonderful patterns, textures, and details of all sorts that're all around us - in nature, in machines, in people's faces, everywhere - which we normally ignore as extraneous stimuli and simply discard any notice of, let alone appreciation. It is a way of seeing (and hearing, and tasting, and generally experiencing sensations of all types and origin) that we have not experienced for a very long time, perhaps not since before we can remember. But to experience the goliath, unyielding continuity of reality with an adult brain and understanding is nothing short of breathtaking.
