Stoicism

3. The foundations of physics

Physics is the study of nature (physis), and to understand the true nature of the world it is necessary to start at the very lowest level of physical analysis. The world and its contents consist of passive ‘matter’ (hylē) and active ‘god’ (theos). These two ‘principles’ (archai) totally interpenetrate each other, and their interaction underlies all change. The active principle is quite literally god, a divine causal force which imbues the entire world with rationality.

Why is the second principle, matter, added? Not in order to make the world corporeal, since god (as well as matter) is already corporeal. It is a fundamental Stoic principle that only bodies are capable of causal interaction, and god could not shape the world if he lacked causal powers. Rather, matter is added because god is an entirely active causal power, and there must therefore be something passive on which he acts. Matter is thus a purely theoretical construct, with no properties of its own to contribute beyond its mere passivity.

Matter and god, then, are conceptual rather than empirical items. At the lowest observable level, at which cosmic processes of change can actually be studied, matter and god are already by their interaction constituted into the traditional four ‘elements’ – earth, water, air and fire. Of these, air and fire form an active and pervasive life force called pneuma or ‘breath’, which by its presence in things constitutes their qualities (see Pneuma §2). Earth and water, on the other hand, are essentially passive elements, which serve a primarily material role: it is only the pervasive presence of pneuma in them that shapes them into complex items, including living things. Thus at the lowest phenomenal level earth and water take over the role which pure matter had held at the lowest theoretical level, while air and fire, paired as pneuma, take over a role analogous to that of the second principle, god.

Pneuma, as the combination of warmth and breathed air that is fundamental to animal life, had already earned a central place in medical theory, to which Stoic physics was heavily indebted. The distinctive contribution of Stoicism, at any rate by the time of Chrysippus, was to extend the explanatory role of pneuma beyond individual animal life, and to make it the vital power of the world as a whole. Since the Stoic world (following Plato among others) is itself a living creature, this extension was not as surprising as it may at first seem. Pneuma is the vehicle of the divine ‘reason’ (logos) which pervades and governs the entire world (see Logos §1).

Pneuma is all-pervasive, but varies in its properties according to its degree of tension. In its most highly attuned form, a portion of it may serve as the soul (psychē) of an animal (see Psychē). Many Stoics held that the human soul, at least, survives the death of the body (although it must of course eventually perish in the conflagration, see §5). A lesser grade of it is called ‘nature’ (physis), in a special sense in which the vegetative life force of a plant may be called its ‘nature’. Finally, a still less refined form of pneuma is present in any discrete object, even a stone or a cup, being what endows it with its cohesion as a single object: as such, it is called the thing’s ‘tenor’ or ‘state’ (hexis).

Both the primary ‘principles’ and pneuma play a key role in securing one of the most characteristic of Stoic tenets, the identification of ‘being’ with corporeality. At Sophist 246–7 Plato had launched a devastating attack on crude materialists who restrict being to bodies. Since they can neither deny that there is (for example) justice, nor equate justice with some body, Plato suggests, they must abandon their position in favour of a new existential criterion, namely that to be is to have the power to interact. Stoicism sets out to defend the materialist position from Plato’s attack: the two criteria considered by Plato are in reality one and the same, since the only things that have the power to interact are bodies. Hence to be is, after all, to be a body.

Virtually everything in the Stoic world is a body (for the few exceptions, see §7–8). This should not be misunderstood as reductive materialism. At the lowest level, ‘god’ is a body, but not a mere body: life and intelligence are his irreducible properties. God has corporeality in addition to these vital properties simply because only thus can he have any causal role in moving and shaping matter. Once this link of corporeality to causality is accepted, it is extended to the vast majority of items traditionally thought incorporeal. Justice, for example, must have causal powers if it is to do any good. It must sometimes make the limbs and voice move in ways in which they would otherwise not have moved. Therefore justice is a body. This apparently paradoxical outcome is defused once we learn what body it is. Justice is simply the soul in a certain condition; and the soul is a body, being an individual portion of pneuma pervading the organism and therefore able to govern its movements. Similarly, (spoken) words are bodies because they are vibrating portions of air, and that is how they can affect the thoughts of a listener.

Stoic corporealism was much derided by ancient critics, but it has the considerable merit of explanatory economy.