Anybody who has some familiarity with the research literature in scientific psychology has probably thought, at one time or another, "Well, all these means and correlations are very interesting, but what do they have to do with me, as an individual person?". The question, innocuous as it may seem, is a deep and complicated one. In contrast to the natural sciences, where researchers can safely assume that, say, all electrons are exchangeable save properties such as location and momentum, people differ from each other. Furthermore, it is not obvious that these differences can be treated as irrelevant to the structure of the organisms in question, i.e., it is not clear that they can be treated as "noise" or "error". The problem permeates virtually every subdiscipline of psychology, and in fact may be one of the reasons that progress in psychology has been limited. As Lykken (1991, pp. 3–4) hypothesizes: