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Descartes scientific theories about the natural world were grounded in a metaphysical foundation, in turn deduced by the application of his rationalist methodology. He wrote that "the whole of philosophy is like a tree. The roots are metaphysics, the trunk is physics, and the branches emerging from the trunk are all the other sciences, which may be reduced to three principle ones, namely medicine, mechanics, and morals".[[CiteRef::Clarke (1992)|p. 271]] One ought to construct a metaphysics first, based on criteria independent of observation, and subsequently consider physical theories consistent with the metaphysical foundation. His natural philosophy was in stark contrast to the accepted Aristotelianism. In Aristotelian natural philosophy all objects were a compound of form and matter, a concept called hylomorphism. Form gives material bodies their distinctive properties, and makes them different from one another. It explains why fire rises and stones fall. Matter is what all material bodies share in common. All things have teleological goals or purposes. [[CiteRef::Shields (2016)]]

In Descartes' mechanical corpuscular natural philosophy, by contrast, there are just two kinds of substance that are entirely different from each other in kind: mental substance and physical substance. The fundamental property of '''mental substance''' was thought, and Descartes equated it with the rational soul of God and humans. The fundamental feature of '''physical substance''' was extension in space. He rejected Aristotle's distinction between form and matter, including Aristotle's four elements. [[CiteRef::Ariew (1992)]] Cartesian mechanics rejects the void posited by atomists; instead matter fills the universe as a plenum. If all matter is extended, Descartes reasoned that there can be no space without extended matter. Also unlike atomism, matter is infinitely divisible, though visible things are composed of tiny corpuscles that interact with one another by physical contact. The corpuscular composition of a material body, rather than its Aristotelian form, determines its properties. Since corpuscles are too small to be directly observed, their size and shape must be hypothesized, though observation can allow us to infer the plausibility of our guesswork. Our senses, Descartes maintained, do not inform us of the mechanical world as it is, but provide us with sensations which are mere signs of their objective causes. Only extended matter and motion exist apart from our minds. [[CiteRef::Clarke (1992)]] Descartes completed a manuscript that was to be a comprehensive expression of his mechanical natural philosophy, called ''The World''. He withdrew his plans to publish it upon learning of the condemnation of Galileo in Rome in 1633. The work never appeared during his lifetime, but two major fragments, the ''Treatise on Light'', and the ''Treatise on Man'' were published posthumously. The first dealt with physics, and the second put forward a theory of physiology, nervous system function, and the mind/brain relationship. [[CiteRef::Garber (1992)]][[CiteRef::Descartes (2003)]]