This essay traces some of Pierre Duhem's motives for his celebrated "Quine- Duhem thesis" to a specific worry about theory underdetermination that arises within classical mechanics, concerned with the rivalry between Duhem's own thermomechanical approach and the more narrowly "mechanical" treatment pursued by Hertz and others. In the context of the treatments of "physical infinitesimals" common at the time, these two approaches seem empirically indistinguishable. After an exposition of the basic issues, this alleged "underdetermination" is then evaluated from a more (...) modern perspective. (shrink)