 Statistically, a textbook heart, for example, may be very rare indeed. But it is the account of the organ that gets into the physiology textbook. The textbook tells you what a healthy organ is like by reference to an abstraction – an idealized organ. This concept of normality is not justified by appeal to a conceptual analysis that aims to capture intuitions about what’s normal. It draws all its authority from its predictive and explanatory utility: against the background of assuming normal heart function, for example, we account for variation in actual hearts (a particular rhythm, say), by citing the textbook rhythmic pattern (which may be very unusual statistically) and identifying other patterns as arrhythmic. The point of textbook depictions of human physiology is to identify an ideal system that enables us to answer “what if things had been different questions” (Woodward 2003, Murphy 2006). The role of an idealization, in this system, is to let us classify real systems according to their departure from the ideal. So normal human biological nature, in this sense, is an idealization designed to let us impose order on variation.