I must extend some sympathy to Charlton because Aristotle's remarks at the start of Physics I are compressed and gnomic.
▻ SUBJECT-MATTER OF PHYSICS
The subject-matter of Aristotelian physics is, Carlton tells us, 'things which are subject to change, things which are not without matter, and things which have in themselves the source of their changing or staying unchanged' (W. Carlton, Aristotle's Physics, I, II, Oxford : Clarendon Press, 1970, ix. This clearly suggests that Aristotelian physics is concerned with physical things, their states and attributes : physical entities or objects, one might say. The field of inquiry is thus, to a first approximation, entities rather than formulas.
▻ METHODOLOGY : ORDERS OF CLARITY
Aristotle's real analytical object in doing physics is to discover the archai, the ruling principles or origins of things - their formulas as you might prefer to say.
However, if we want to be acquainted through discovery with the archai, we need to take an oblique route. In order to come to know a thing's own essential nature but we have to start with what is clearest and most familiar to us, given our own natures. This means starting with observation (aisthesis) of the physical world; this is what is initially clearest and most familiar to us.
Gradually we are able to apply induction (epagoge) to the data of observation : from observations we can derive universals (kinds), we can work out what a particular thing observed is an instance of, the kind of thing it is. From this in turn we can analyse a kind of thing as a specific type of combination of form and matter; ad from this we can ascertain its essential nature. We discover its
arche, its ruling principle or origin, what makes it the thing it is.
Now, the archai, which make kinds the specific kinds they are, are 'clearer or better known by nature' in the sense that they are primary in determining what kinds there are and what their essential natures are : they are first in the order of explanation of nature. But when we started our inquiry we could not start with them, since they were veiled from us. We had to start from what was clearest and most familiar to us, namely observation of the physical world.
We started with what was clearest and most familiar to us (with what was knowable by us). We did so in order to reach eventually what was initially not clear or familiar at all, the essential natures of things (things known in their fundamental natures).
Aristotle does not use the term, epagoge, at the beginning of Physics, I. I have moved his argument around a bit to make it less puzzling.
Russell Winslow, Aristotle and Rational Discovery, London : Continuum, 2007.
G.E.L. Owen, Logic, Science and Dialectic, Ithaca : Cornell University Press, 1986.
R. Bolton, 'Aristotle's Method of Natural Science : Physics I', Aristotle's Physics : A Collection of Essays, Oxford : Clarendon Press, 1991.