Frequestly Asked Questions (and answers) about Stoic philosophy
Modern people do not generally adopt the classical Stoic views on physics, although understanding it is sometimes helpful in interpreting ancient Stoic writing. The Stoics were materialists, in that they believed that the universe consisted only of corporeal bodies. They thought the universe was a solid sphere of substance surrounded by infinite void. Every point in the sphere has some combination of four elemental substances: earth, water, air, and fire. These substances are not necessarily exclusive: a single point can have a superposition of elements, in a variety of states. Points in the middle of a red-hot piece of iron have a superposition of fire and earth, while a cold piece has less fire but just as much earth, for example.
They also thought the universe could be divided into passive and inert parts, stationary unless being acted upon, and an active part, pneuma. Pneuma extends, at some level, though every point in the cosmos, and is what pushes the other stuff around. It comes in several varieties, which cause different kinds of actions (plant-like, animal-like, rational, etc.). The rational variety is most concentrated in the hearts of living humans, the sun, stars and other heavenly bodies. Because pneuma fills everything, God moves everything by the same physical mechanism humans use to move their limbs, etc.
The relationship between pneuma and the four elements is uncertain, but some classical sources assert that it is a combination of air and fire, and others a particular kind of fire.