To what extent is the scientist's endeavor qua scientist influenced by his philosophic image of himself? A preliminary and partial answer to this question is suggested by a study of eight physiological thinkers of the second half of the eighteenth century, a period during which biology was much influenced by the scientific and philosophical ideas of Isaac Newton. At this time, physiologists invoked certain "principles," "properties," and "powers" which were deemed useful as explanatory devices, even though they could not themselves (...) be explained. The use of these devices in physiology was often defended in terms of their supposed similarity to devices used in mathematics and Newtonian physics. The results of this practice are considered critically, partly in the light of current studies of scientific explanation. (shrink)