The word “physics” can be confusing for modern English speakers when we’re discussing the tripartite divisions of philosophy, to wit: logic, ethics, and physics. Today, Physics requires electron microscopes, crazy-intense lasers, Large Hadron Colliders, and other assorted machines and instruments. Yet, these are pretty thin on the ground.
Most of us don’t have access to such tools, nor did the classical Stoics. So why do we call their study ‘Physics’ also? The operative word in the Koine is Physis ( φύσις), and is commonly translated by the English word ‘nature.’ It has philosophical, theological, and scientific connotations. We use the same word in English, it’s derivative “physics,” because we’re talking about similar intents: the desire to study nature, or reality.
The classical study of Physics incorporated things that we might categorize as theology, cosmology, psychology, anthropology, biology, chemistry, etc. It’s quite the range of areas of investigation. But the crucial point is that they are investigations into the nature of the cosmos: of reality.
Metaphysics is a newer term, and it’s often applied retroactively to thought-models which are deemed to be outdated or untestable. The morpheme ‘meta-‘ in English has the meaning of “beyond” or “above.” So metaphysics postulates about things which are beyond current conventional ability to test. While it might be acceptable in some academic disciplines to refer to certain positions of the classical Stoics as metaphysical, we who consider ourselves studying in the school usually will make use of the word “physics” as it’s closer to the vocabulary which they themselves used.
So, how come the Stoics don’t lay out statements like “Here’s our metaphysical position on ‘X-thing,'” ? The first reason being, the term is new, so we’re not going to see it per the above. The second is that the Stoic worldview is interwoven into the entire system. By the time the classical Stoics were current, philosophy had become a system of schools, which had held common positions amongst themselves. Of course, there are those who make individual contributions to the discussions, and some of them are heterodox to the mainline dogma: nothing surprising there. Thirdly, using the word “physics” places the things we’re discussing in a chronological context. To understand modern science in the best way, it is advisable to at least learn about the understandings of previous generations. To really understand why quantum mechanics is such a trip, esp. in the 1905, a firm understanding at some level of Newtonian physics is useful. Context matters.
The classical Stoics are variously called materialists, vitalists, monists, physicalists, and more. What we’re discussing here is not a proclivity for shopping, but rather their understanding of the cosmic nature. The classical Stoics believed (or at least espoused) that all of existence is made up of one stuff, that it’s ordered by a universal reason, and that virtue is the only good and equivalent to eudaimonia.
These are metaphysical positions generally, as we understand it.
However, the Stoic ethics are predicated on its physics. While recently this position has been challenged by some, the academic literature and the classical sources themselves are (to me at least) clear on this issue. As such, to relegate them to the realm of the “metaphysical” does a disservice to the unity of the system for the modern student.
This is precisely because the classical position gives us an avenue for the modern practitioner to approach life. If it is possible to divine ethical precepts based on a rational understanding of the universe (assumption), then we still have work to do. The School is in progress. The case of ethics is not closed. The understanding of virtue is not closed. These are open classes, and it is our responsibility as philosophers to continue that work.
Whether you call a certain sub-set of classical postulates and beliefs “physics” or “metaphysics” isn’t really an important-per-se issue; nor is it of a moral nature (thus an indifferent). Within some circles one might be more prevalent than the other, the entirety and the length of the discussion is still one of value, however.
You won’t be kicked out of the Stoic tent (so far as I’m concerned) if you do not believe that universe is all matter surrounded by a cosmic void, or that it forever expands and is consumed in a cosmic fire. You don’t have to believe that the soul of the Sage lives on for a short time (but not past the Ekpyrosis). But it is important to be familiar with the classical beliefs that came along with the ethical and moral precepts of our School if you are to call yourself a Stoic.