 For similar reasons, one should not reject Descartes’ assumptions about parts of matter in motion in the human body (such as animal spirits or the nerves) simply because, given their size, they are unobservable. Although Descartes and La Forge were attempting to describe parts of the human body and their motions before the invention of the microscope, the methodological issue involved would not have changed even if the threshold of observability had been modified by optical instruments: the Cartesian theory of explanation supported the postulation of unobservable particles of matter to explain visible effects whenever they were thought necessary. Their explanatory role justified their hypothetical postulation.