At the beginning of Part III, Descartes describes his project as that of explaining ‘all the phenomena of nature’ by means of the principles already set out in Part II, beginning with ‘those phenomena that are the most universal and on which the rest depend’ (art. 1). In connection with the realisation of this project, Descartes sets out two fundamental principles. The first is that we must not assume we know the ends for which God created the world (art. 2), and, in particular, we cannot assume that God created the world for human ends: ‘it is in no way likely that all things were made for us in the sense that God had no other purpose in creating them’ (art. 3). The second principle is that many more things follow from the laws set out in Part II than are evident in our universe (art. 4): our actual world is not the only one compatible with these laws.

These principles do not state commonplaces, and they engage two fundamental questions: curiosity and teleology. The question of curiosity was a highly charged one throughout the Middle Ages and the Renaissance. Augustine had argued that since heavenly bodies were mentioned only rarely in Scripture, the study of astronomy was of very limited use in dealing with scriptural questions, and was likely to distract one, and hence the most beneficial and honourable course was to desist from astronomy altogether.