- The Stoic Psychological Physicalism:An Ancient Version of the Causal Closure Thesis
§1. Introduction: Stoic Physicalism and the Platonic Tradition
Physicalism lies at the very core of Stoic ontology; the view that something is actually real if and only if it is corporeal is so strong in the Stoic sources that the suspicion that the Stoics must have endorsed physicalism in some of its forms looks like a very promising approach. 1 If the general Stoic physicalist tenet is analyzed within the psychological domain, one might be tempted to side Stoic physicalism with the mind-body identity theory, that is, the contemporary view in philosophy of mind that is willing to advocate the identification of mental states (and events) with the physical processes in the brain. Indeed, the Stoics do not go so far as to claim that there is an identity between mental states and some physical processes in the brain. As a matter of fact, they wrongly took the "mental events" to take place not in the brain, but in the heart. 2 However, the account is structurally similar as they considered that mental states are corporeal and that, thereby, they have [End Page 105] a seat in a specific organ of the body. Furthermore, the Stoics are disposed to argue that this fact is possible because both psychological items and the heart are bodily.
The Stoics, like many contemporary philosophers of mind, were monists (all that exists is matter, or rather, "what is corporeal") 3 ; they also believed, like contemporary physicalism, that the physical world is self-contained and explanatorily self-sufficient (Kim 2005, 154), and thence vigorously refused substance dualism and the possibility that immaterial items (such as Platonic forms and ends) can have causal power. Of course, these can only be incidental coincidences; however, if my suggestion that Stoic psychological physicalism constitutes an ancient version of the "causal closure thesis" (hereafter CCT)—that is, the thesis that the physical world is a domain causally closed, or to put it differently, what is physical can solely have physical causes 4 —is at least plausible, I need to show that such coincidences are not "mere coincidences," but instead that they strongly derive from and are grounded on systematic aspects the Stoics were interested to defend. I can only conjecture the motivations they had for refusing the Platonic-Aristotelian ontology; in the psychological domain, though, the Stoic dissatisfaction regarding the Platonic dualism and the soul-body problem must probably be sought in Plato's account of the manner in which the soul, being an immaterial item, is able to produce some effect on a material body.
As we shall see in §3, a relevant premise of the Stoic arguments against substance dualism is focused on the fact that between immaterial and material items is an incommensurable gap in which causal action of immaterial things upon the bodies is impossible. Certainly, if this is exactly the case, this criticism applies to any immaterial item (including forms and ends) seen as having causal power. This kind of dissatisfaction is not essentially different from that of contemporary physicalism, which, at least in part, can be seen as a sort of reaction to some forms of what might be called folk psychology, that is, those views that take for granted that mind and body are two distinct things in character, and that in any case, the mind has causal power over the body, neglecting the fact that mental states cannot be understood independently of the brain (Papineau 2001, 4). Although the Stoics do not say that psychological states can be identified with the brain (32) or any other [End Page 106] sense organ (see note 2), they forcefully argue that soul and body interact, but in order to interact they must be of the same nature. Otherwise, the already mentioned gap cannot be overcome.
Without going so far as to say that the Stoics would have endorsed the mind-body identity theory, I think one might suggest that the difficulties the Stoics found in substance dualism were probably noted even by Aristotle himself, who nevertheless was willing to support dramatically the thesis that (soul) and psychological events were...