Diogenes Laertius Vitae philosophorum 7.1.148

[148] The substance of God is declared by Zeno to be the whole world and the heaven, as well as by Chrysippus in his first book Of the Gods, and by Posidonius in his first book with the same title. Again, Antipater in the seventh book of his work On the Cosmos says that the substance of God is akin to air, while Boëthus in his work On Nature speaks of the sphere of the fixed stars as the substance of God. Now the term Nature is used by them to mean sometimes that which holds the world together, sometimes that which causes terrestrial things to spring up. Nature is defined as a force moving of itself, producing and preserving in being its offspring in accordance with seminal principles ¹ within definite periods, and effecting results homogeneous with their sources.