Sober on Essentialist Explanations in Science

"My suggestion is that the gas's kinetic energy explains what it is for the gas to have the temperature it does. It does not explain why the gas has the temperature at the time in question. The relationship here is much like the one between a dispositional property and its physical basis. If zebras differ in fitness because of their leg structure, then there is a sense in which leg architecture explains fitness relationships. The relationship is not causal; it does not explain why those fitness values came into existence. Science not only explains why certain states of affairs and events come into existence; it also seeks to explain the nature of those events."

--Elliott Sober, The Nature of Selection, p. 75.

Note that explanation is hyperintensional, and that not just any necessarily coextensive term would work equally well in place of 'the gas's kinetic energy' here.