It’s natural to us to divide the objective and subjective, the exterior and public world from the interior and private one. But the division is not natural in the sense of being universal – trying to find it before Descartes is not an easy thing, still less finding it as a fundamental division of being.
Descartes divided thinking substance from exterior, extended substance by an epistemological criterion: we cannot be deceived about interior reality but we might be deceived about exterior reality. The criterion continues to this day from Russell to Chalmers arguing that consciousness is a primary thing known.
The argument is a strange one since, taken on its own terms, we get only a division of how things are known to us. The Cartesian argument leaves open the possibility that the exterior and interior worlds are really the same. Perhaps they’re one thing, or two things, or infinite things, or nothing at all; all we know is that, in the face of whatever-it-is we’re certain of one thing and less certain of another. In seeking to escape skepticism we end up with a distinction in things that leaves the world unknowable.
The other consequence is that the exterior world, by definition, lacks interiority. It becomes essentially intert, having no source within itself to give rise to activity. At the same time, the interior world is one of pure consciousness, that is, a world where all causal relations are logical inferences. But no one thinks that a cannonball brings down a wall by a logical inference. The exterior world is thus lifeless (mechanical) and causal power becomes only logical. What makes things go is a “law of nature” which is either in no way causal but only a description of what happens, leaving all real causes unsaid (the hard headed Empiricist tack) or it is endowed with a mystical mathematical causality acting within things (the idealist tack). But the dilemma arises only because we’re committed to holding that causal power is interior to things, and what is extrinsic has no interior.
Aristotle’s idea, as developed though Medieval thought (both Dominican and Franciscan) is that all things are, as it were, blooms proceeding from some source or principle. The first division in things is in the source that gave rise to them, that is, what we are given in the world is not an appearance, but a fulfillment of some arche or principium. One such arche is extrinsic to things, which gives us a class of the artificial and, by inference, a mind as the proper instrument giving rise to it. This is a totally different division of “the interior” and “the exterior”, but at least one that as the benefit of not confusing the map with the territory.