"Metal" only refer to a set of kinds of matter. A set we shaped because it helps us make useful inferences without using too much brain power. Like the fast rules: "Most metals are good conductors", "Most metals are strong", "Most metals are hard", "Most metals are heavy".

Then someone comes and shows you that new material called "glass" that is heavy, hard, and strong (this one is bullet proof). You'd be quick to infer that it is a metal, and therefore probably a good conductor. But didn't we tell you that most metals are opaque?

http://lesswrong.com/lw/no/how_an_algorithm_feels_from_insid...