As a young student in Copenhagen, Nicholas Steno was well acquainted with the work of the French philosopher and scientist René Descartes and adopted his methodological principles and many of his theories as well. Empirical anatomical research on glands, muscles, and brain gradually made Steno more and more critical of Descartes’ stringent but wholly deductive reasoning. Nevertheless, on a deeper level, most of Steno’s path-breaking research in anatomy, as well as in geology, operated within a mechanist Cartesian framework, and several of his path-breaking discoveries can be linked to his testing of crucial points in the Cartesian theory of how the “human machine” works. It might be a little too much to cast Steno as a Popperian avant la lettre, but viewed from a Kuhnian perspective, Cartesianism can aptly be described as Steno’s research paradigm.