Summary Robert Boyle is remembered largely for his integration of experiment and the ?mechanical philosophy?. Although Boyle is occasionally elusive as to what he means precisely by the ?mechanical philosophy?, it is clear that a major portion of it concerned his corpuscular theory of matter. Historians of science have traditionally viewed Boyle's corpuscular philosophy as the grafting of a physical theory onto a previously incoherent body of alchemy and iatrochemistry. As this essay shows, however, Boyle owed a heavy debt to (...) a longstanding alchemical theory that postulated the corpuscular make-up of metals and various reagents. I have elsewhere argued in a general way for Boyle's debt to alchemical corpuscular theory, but in the present essay I show one of his precise sources?namely the Wittenberg medical professor Daniel Sennert (1572?1637). In his youth Boyle wrote a treatise on corpuscularianism, Of the Atomicall Philosophy, that borrowed heavily from Sennert without acknowledgement. Later works, such as The Sceptical Chymist, elaborate on these borrowings. This discovery shows that we cannot view Boyle's corpuscular philosophy as an imposition of physics on chemistry: instead, it appears that it originally grew out of chemistry itself. The discovery also throws an interesting light on Boyle's attitude toward sources. (shrink)