We can only manage five

"A series of psychological experiments, many dating back to the 1950s, shows that we cannot distinguish between more than about five degrees of … well, almost anything: sweetness in a solution; saltiness; the pitch of a note; brightness; the intensity of an electric shock; the length of a line; or the pungency of a smell. The details vary, but the level of consistency is surprising...Practice does not help (more from Tim Harford)"