Daniel Sennert , a prominent physician and a prolific and influential writer, was both an atomist and an Aristotelian. He was influenced by a distinctive and now little known Aristotelian approach to matter and form, and this promoted his development over time of a hierarchical account of atoms, with elementary atoms and grades of molecules. The first section provides a study of Sennert's Aristotelian foundation. The final two sections consider, in turn, Sennert's development over time of an atomistic theory, and (...) the nature and significance for him of substantial forms. (shrink)