Descartes is not usually associated with an empirical, let alone experimental, approach to science. According to the textbook version of Descartes’ philosophy, the fundamental principles of Cartesian physics are established independently of all experience; all one has to do would be to identify them through a process of systematic doubt and to justify them with an appeal to divine veracity. Although this idea of Cartesian science is not entirely false, there is considerable room for qualification. Moreover, Descartes’ correspondence shows that, despite a deep distrust in the observations and experiments of others, Descartes rarely rejects them as being irrelevant, usually asks to be informed of the precise circumstances, and quite often suggests new experiments of his own. The bottom line, however, is that for him no observation or experiment can ever be useful without a correct general theory; without it, experiments are mere “curiosity,” which, being a kind of wonder, is of little use in science (AT X 371, CSM I 16) and even “perverts the use of reason” (AT XI 385–86, CSM I 355–56).

The Discourse is one of the few places where Descartes explicitly discusses experiments and affirms their necessity. Descartes situates himself in a distinctly Baconian perspective, emphasizing both the practical and, in a limited way, the collective aspects of science. He also admits the necessity of experiments but is eager to point out their limitations. Observations and experiments are necessary “the more we advance in our knowledge” but not when we begin: “If we begin, instead of seeking those [observations] which are more unusual and rather contrived it is better to take those only into account which, presenting themselves spontaneously to our senses, cannot remain unknown after a little reflection” (AT VI 63, CSM I 143). Unusual observations (which includes experiments) would be misleading if we did not know the explanation of the more general ones.

As a result, Cartesian natural philosophy, or physics, consists of two levels: one dealing with the most general features of nature and based on ordinary experience, and the other dealing with particular effects. The first level, general physics, in turn, can be divided into two parts: the derivation of “the principles or first causes of everything” from “seeds of truth that are naturally in our souls”; and an examination of the “first and most ordinary effects that can be deduced from those causes” (AT VI 64, CSM I 144).