Аннотация:Abstract Daniel Sennert (1572-1637), 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.