The Stoics divided philosophy into logic, physics and ethics, and were fond of images insisting that the three cannot be separated. Philosophy is an orchard, where logic is the wall, physics the trees and ethics the fruit; or an egg, where logic is the shell, physics the white and ethics the yolk; or an animal, where the parts are its bones, its flesh and its soul. The point of the images is that the ethics does not stand up without the physics.
The physics is a thoroughgoing materialism with a divine principle inside it rather than above it. There is one substance, pervaded and held together by pneuma, a fine active breath whose tension gives things their coherence and whose highest degree is reason. That pervading reason is the logos, and the Stoics were willing to call it God, fate, providence or nature depending on the aspect under discussion. The cosmos is therefore a single living being that thinks, and everything in it follows from what has gone before.
At intervals the whole is resolved into fire and begins again, in some accounts identically, which gives the system its doctrine of recurrence. A human being is not a spectator of this order but a fragment of the reason that runs it, which is the ground of the ethics: to live consistently with nature is to live consistently with oneself, because the two are the same thing seen from different distances.