Cartesian Certainty, Realism and Scientific Inference

In the Principles, Descartes explains several observable phenomena showing that they are caused by special arrangements of unobservable microparticles. Despite these microparticles being unobservable, many passages suggest that he was very confident that these explanations were correct. In other passages, however, Descartes points out that these explanations merely hold the status of ‘suppositions’ or ‘conjectures’ that could be wrong. The aim of this chapter is to clarify this apparent conflict. I argue that the possibility of natural explanations being wrong should be understood as these explanations not being absolutely certain, but as being morally certain. Cartesian explanations rely on what Ernan McMullin calls retroduction, which is a mode of inference that justifies beliefs in concrete unobservable entities and processes. I use as a foil the debate in contemporary philosophy of science between scientific realism and instrumentalism, and argue that for Descartes we could indeed have knowledge of the unobservable world. In that sense, he was closer to being a scientific realist.