In part 5 of "Discourse on the Method" (titled: " Physics, the heart, and the soul of man and animals ") René Descartes moves from the metaphysics of part 4 to physical considerations. Descartes was hesitant in previously publishing his thoughts on the matter following Galileo's contamination over his heliocentric views, but now he feels more confidant in their acceptance.
Descartes first discusses light and its natural origins, considering celestial bodies as conveying light and humans as the perceivers of that light. He then discusses matter, saying that God's laws must be perfect and therefore imperative in any possible world (not just ours). These laws separate, according to Descartes, matter into bodies which operate in consistency with these laws. What's important to note here is not so much Descartes' physics but rather the notion to natural law, originating in God, can account for all natural phenomena. If we, let's say, take God out of the equation nothing will change. One way to see this is to argue that Descartes holds up the "God as watchmaker" view, meaning that God has created the laws which govern the world without his interference. Another possible interpretation is much more radical and could at the time come at a very high cost for Descartes who tip toes around the notion of a purely rational, none-religious, account of natural order. Descartes avoids heresy by claiming that while nature can be understood through natural law the capacity of the human soul for reason must be Godly. Reason is what for Descartes separates us from animals. Reason and its subsequent use of language is what makes humans, unlike animals, act on more than just instinct. This, for Descartes, proves that our souls come from God and are in fact immortal.
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