Descartes was among the first modern scientists to make the search for laws of nature central to his scientific work, and apparently the first philosopher to think deeply about the logical and metaphysical status of these laws, developing a theory that provides a significant alternative to the Humean approach that long dominated Anglophone thinking on this topic (Milton 1998, 686).

Writing to Mersenne in November 1629, Descartes announces an ambitious project: he has resolved to explain, not just one phenomenon, but “all the phenomena of nature, that is, the whole of physics” (AT I 70, CSMK 7). In The World, begun early in the 1630s, but never finished, we see the beginnings of that project. Descartes invites us to imagine a new world, somewhere in “the imaginary spaces,” where God created so much matter that in whatever direction our imagination extends, it finds no empty place (AT XI 31–32, CSM I 90). In this matter are three kinds of elementary particles, distinguished only by size, shape, and motion (AT XI 34, CSM I 94) (see element). The phenomenal properties of macroscopic objects (heat, light, hardness, fluidity, etc.) are to be explained by the properties of their constituent particles, and especially by motion, according to the laws of nature.

This early mechanist program posits three fundamental laws:

1. 1.Unless an encounter with other bodies causes a change, each part of matter continues in the same state: size, shape, and position (if at rest), or motion (if in motion).

2. 2.When one body collides with another, it cannot give it any motion unless it loses as much as the other body gains; if it takes motion from the other body, it must gain as much motion as it takes away.

3. 3.If a body is in motion, even if it usually moves in a curved line, each of its parts will tend to move in a straight line, departing from that path only as constrained by other bodies. (“Usually” is odd. Since Descartes rejects any vacuum, he holds that all actual motion must be in a closed curve.)(AT XI 36–48, CSM I 92–98)