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To satisfy that equation, a gas would have to consist of perfectly elastic spherical particles of negligible volume and exhibiting no mutual attraction. Z. Elgin and attraction. With these parameters fixed, the interdependence of the values of temperature, pressure, and volume is exemplified. The law and the model it defines are fictions. There is no such gas. Nevertheless, the model advances our understanding of gas dynamics. It exemplifies a relation that is important, but hard to discern in the behavior of actual gases.

Drastically limiting the factors that affect her protagonists enables her to elaborate the consequences of the relatively few that remain. If our interests are cognitive though, it might seem that this detour through fiction is both unnecessary and unwise. Instead of resorting to fiction, wouldn’t it be 3 See Cartwright (1999, 77–104). Z. Elgin cognitively preferable to study three or four real families in a real country village? Probably not, if we want to glean the insights that Austen’s novels afford.

I shall here repeat an earlier answer of mine, that these models are like fables, for instance like this fable that I shall discuss below: A marten eats the grouse; A fox throttles the marten; the tooth of the wolf, the fox. Moral: the weaker are always prey to the stronger. Like the characters in the fable, the objects in the model are highly special and do not in general resemble the ones we want to learn about. Just as I have never seen a frictionless plane or a worker interested only in leisure and income, I don’t think I have ever seen a marten, and seldom a wolf.