The idea of combining molecular mechanics with statistical principles, in order to provide a “reduction” of thermodynamics to mechanics, took shape about a hundred years ago. Since that time, the technique of explaining macroscopic properties of matter in microscopic terms has advanced by leaps and bounds; but this practical success is not matched by a comparable progress in our understanding of the “foundations”. Recent mathematical advances in the ergodic theory ([18] and the various papers published in [6]) make it only more apparent that such discussions are still pursued in the conceptual framework of Boltzmann. My purpose is to advance another approach that is in some sense complementary to the traditional one. I suggest first a heuristic guideline that is broad enough to subsume both programs.

Most of what I have to say concerns Tisza's conceptual analysis [12], but I should like to begin with an appreciation of Brush's historical essay [2], Accurate historical information about a science is usually illuminating not only about the process of discovery but also about the subject matter itself. I think that this statement is especially true for statistical mechanics, in which the problems that the founding fathers were concerned with are still not completely solved, and I certainly feel that this historical essay has helped me to understand the subject matter of statistical mechanics better than before.

There are several remarks in Brush's essay concerning epistemology and methodology, some of which seem to me correct but others not.