In each of the decades of his maturity, Descartes embarked upon an unfinished project: the Regulae in the 1620s, Le Monde/L'Homme in the 1630s, and the Principia in the 1640s. The first two of these projects inaugurate major changes of direction in Descartes' thinking, while the third attempts to consolidate a major development begun in La Discours de la M'ethode and the Meditationes. There are some themes that persist, however, and this is particularly true of Le Monde/L'Homme, which provides much of the material for the final project. Indeed, in thinking through this final project, Descartes talks of teaching Le Monde ‘to speak Latin’ before bringing it into the world, and ‘naming it Summa Philosophiæ to make it more welcome to the Scholastics, who are now persecuting it and trying to smother it before its birth’.

Between the abandonment of Le Monde and the publication of the Principia, Descartes formulated some of his results in method, optics, meteorology, and geometry in the form of four essays, published in 1637, and then he turned away from explicit natural philosophy for a while. Developing a theme that had already been evident in the first of these essays, La Discours, he set out a sceptically driven epistemology as a way of indicating the tasks of a foundational metaphysics in the Meditationes. Then, ‘when I thought that these earlier works had sufficiently prepared the minds of my readers to accept the Principia Philosophiæ, I published these too’.