Ontology

For the Stoics, physics is that part of philosophical discourse that deals with all questions concerning the physical world, from foundational ontology to the empirical sciences such as astronomy and meteorology. The fundamental assertion underpinning all of Stoic physics is the claim that only bodies exist, a claim that dates back to Zeno himself. This may be seen as a direct challenge to the Platonic claim that the material world that we experience is merely a shadow of another realm where real existence lies. Indeed, it echoes a position attributed to one of the parties in the discussion of the nature of existence in Plato's dialogue the Sophist (on this see Brunschwig 1988).

In the Sophist Plato mounts a famous attack on materialism as an ontological position (245e–249d). He refers to a battle between giants and gods over whether “being” pertains only to physical objects or whether it pertains to non-physical entities. The materialist giants insist that being is “the same as body”. Anything that they cannot touch or squeeze in their hands, as they can with bodies, does not exist at all. Thus they must deny the existence of non-bodily entities such as soul, intelligence, justice and virtue. For Plato, these conclusions are not only unpalatable but also probably disingenuous, for no one seriously denies the reality of these things. This extreme position is tempered, Plato suggests, by more moderate materialists who claim that the soul does exist but that in order to exist it must be a special kind of body.