So, a substance has capacities. This is important for Galen because it provides him with a way of formulating his natural philosophy in opposition to either Atomism (random motions of bodies which are themselves indistinguishable in their properties) or mechanism, in particular the view attributed to Erasistratus that all processes in the body can be causally reduced to the mechanical principle of horror vacui (pros to kenoumenon akolouthia)—a view attacked at length in Nat. Fac.