In Stoic physics, blending (κρᾶσις) is the relation between active pneuma and passive matter; natural bodies from rocks and logs to plants, animals and the cosmos itself are blends of pneuma and matter. Blending structures the Stoic cosmos. I develop a new interpretation of the Stoic theory of blending, based on passages from Hierocles. The theory of blending, I argue, has been misunderstood. Hierocles allows us to see in detail how the theory is supposed to work and how it fits into Stoic physics.

According to the Stoics, ordinary unified bodies—animals, plants, and inanimate natural bodies—each have a single cause of unity and being: pneuma. Pneuma itself has no distinct cause of unity; on the contrary, it acts as a cause of unity and being for itself. In this paper, I show how pneuma is supposed to be able to unify itself and other bodies in virtue of its characteristic tensile motion (τονικὴ κίνησις). Thus, we will see how the Stoics could have hoped to account for corporeal unity by positing another body (pneuma) apparently itself in need of unification.

This paper considers what function—if any—colocation of bodies may have in the Stoic theory of blending (κρᾶσις), by examining (1) whether colocation is part of the definition of what blending is; and (2) whether colocation is posited by the Stoics as a requirement necessary for the definition to be satisfied. I reconstruct the standard, Chrysippean definition of blending, and I show that the answer to (1) is ‘no’; further, I argue that the evidence gives no reason to affirm (2). Thus, it appears that colocation does not have a central function in the Stoic theory of blending.