  Scholars have proposed various schemes for dividing Descartes' life into periods. This entry adopts a relatively simple division between the era when mathematics provided the model for his method and the period after the “metaphysical turn” of 1629, when his conception of the role of the intellect in acquiring knowledge changed and when he came to conceive the truth of his special or particular hypotheses in natural philosophy as less than certain and so as subject to the scheme of confirmation through consequences mentioned above. In effect, he adopted a hypothetico-deductive scheme of confirmation, but with this difference: the range of hypotheses was limited by his metaphysical conclusions concerning the essence of mind and matter, their union, and the role of God in creating and conserving the universe. Consequently, some hypotheses, such as the “substantial forms” of scholastics, were ruled out. Argumentative differences among the World, Discourse, and Meditations and Principles may then be seen as arising from the fact that in the 1630s Descartes had not yet presented his metaphysics and so adopted an empirical mode of justification, whereas after 1641 he could appeal to his published metaphysics in seeking to secure the general framework of his physics.