One question we understand today is this: what are the fundamental entities
of which the universe is made up? I don't say we understand very clearly what
we are asking, but we understand enough to know that 'atoms' is an outline answer
to it, which we probably think is now discredited. We may think 'packets of
energy' could be rival answer, or 'waves', or 'wavicles'.
This is a question physics sets itself to answer, and it is part of what Descartes was interested in.
If we are to identify physical laws, we must know what the subjects of those laws are to be. Is physics to have lots of laws beginning eg This object ...; or This type of object (eg horse), or Matter ... That is, what are we going to take as the basic things in the universe, the things in terms of which the laws of physics are to be framed?
The revolution Descartes was bringing about was as much in physics as in how the human being was to be regarded. The physics of the scholastic period had been Aristotelian. Material things had natures, and tended to move in the direction their natures commanded. Part of a thing's nature was to be 'at home' in a particular region of the universe: fire above water, water above earth, for example. If a thing were displaced from its 'natural place' it had within it a tendency to move towards that place. Water runs downhill, fire rises, rocks thrown into water sink.
This ascribed to things the power to initiate action.