Stoic physics refers to the natural philosophy of the Stoic philosophers of ancient Greece and Rome which they used to explain the natural processes at work in the universe.
To the Stoics, the cosmos is a single pantheistic god, one which is rational and creative, and which is the basis of everything which exists. Nothing incorporeal exists. The nature of the world is one of unceasing change, driven by the active part or reason (logos) of God which pervades all things. The active substance of the world is characterized as a 'breath', or pneuma, which provides form and motion to matter, and is the origin of the elements, life, and human rationality. The cosmos proceeds from an original state in utmost heat, and, in the cooling and separation that occurs, all things appear which are only different and stages in the change of primitive being. Eventually though, the world will be reabsorbed into the primary substance, to be consumed in a general conflagration (ekpyrôsis), out of which a new cycle begins again.
Since the world operates through reason, all things are determined. But the Stoics adopted a compatibilist view which allowed humans freedom and responsibility within the causal network of fate. Humans are part of the logos which permeates the cosmos. The human soul is a physical unity of reason and mind. The good for a human is thus to be fully rational, behaving as Nature does in the natural order.