A short version of this history runs like this: (1) the mechanistic physics of the seventeenth century allowed only a very narrow range of such causes; (2) early Newtonian physics was more liberal, and indeed did not impose any real restrictions on possible causes of physical effects; (3) however, the discovery of the conservation of energy in the middle of the nineteenth century limited the range of possible causes once more; (4) moreover, twentieth-century physiological research has arguably provided evidence for yet further restrictions.