Many of us are prone to lay-philosophizing about the weird privacy of our own mental states, for example, and from the fact that when my knee hurts only I can feel it, it’s tempting to conclude that for me the word pain has a very subjective internal meaning that only I can truly understand. This line of thinking is sort of like the adolescent pot-smoker’s terror that his inner experience is both private and unverifiable, a syndrome that is technically known as Cannabic Solipsism. Eating Chips Ahoy! And staring very intently at the television’s network PGA event, for instance, the adolescent pot-smoker is struck by the ghastly possibility that, e.g., what he sees as the color green and what other people call “the color green” may in fact not be the same color experience at all: The fact that both he and someone else call Pebble Beach’s fairways green and a stoplight’s GO signal green appears to guarantee only that there is a similar consistency in their color experience of fairways, and GO lights, not that the actual subjective quality of those color experiences is the same; it could be that what the ad. pot-smoker experiences as green everyone else actually experiences as blue, and what we “mean” by the word blue is what he “means” by green, etc., etc., until the whole line of thinking gets so vexed and exhausting that the a.p.-s. ends up slumped crumb-strewn and paralyzed in his chair.
-David Foster Wallace