The idea of reduction in the empirical sciences is at least as old as the mechanical philosophy of the seventeenth century, which in effect simply required that all physical phenomena be explained by, or “reduced” to, local contact interactions between impenetrable particles of matter. The properties that were attributed to these particles were size, shape, motion, and sometimes gravity. On this basis all physical properties of bodies – such as mass, weight, or their ability to reflect, refract, absorb, or polarize light – as well as all their chemical properties, were to be explained. By the middle of the nineteenth century it became clear that the mechanical philosophy – as originally conceived – could not provide an adequate basis for physical theory. However, the program of accounting for the theories, laws, and empirical facts in one scientific domain on the basis of those in another, that is, “reducing” the former to the latter, continued to play a significant role in scientific research. In fact, it was in the nineteenth century that this program began to achieve some important successes. Maxwell's theory of electromagnetism provided an implicit reduction of the laws of geometrical optics to what may be called “physical optics.” Maxwell, and especially Boltzmann, attempted to reduce the thermodynamic laws to the kinetic theory of gases. Meanwhile, Helmholtz and his associates developed an experimental program to found biology upon physical principles. These programs achieved at least partial success.