In the Ethics Spinoza attempts to give his philosophical metaphysics an absolute foundation in the style of Euclidean proofs. One of the principal focuses of the Ethics is to show that God is Nature and Nature is God. Like Descartes and Leibniz, Spinoza was a rationalist which meant he believed that all knowledge could be deduced from certain a priori self-evident truths.
The ancient Stoics would agree with Spinoza that God is Nature and Nature is God. The ancient Stoics went about many different means to prove the existence of God as a reasoning Universe. Zeno of Citium used something similar to the ontological argument to prove the Universe was a reasoning being. Zeno declared:
That which exercises reason is more excellent than that which does not exercise reason; there is nothing more excellent than the universe, therefore the universe exercises reason.
Spinoza reasoned that everything was God because God was a substance with infinite mental and material attributes. Everything owed its existence to God because all of our mental attributes and material attributes were all part of God. We were all tied together with the substance of Nature of God itself.
According to Spinoza, while we were absolutely determined both mentally and materially to be as we are, God or Nature was self-determined because God was a being from which all cause and effect relationships would arise.
From God’s supreme power, or infinite nature, an infinite number of things – that is, all things have necessarily flowed forth in an infinite number of ways, or always flow from the same necessity; in the same way as from the nature of a triangle it follows from eternity and for eternity, that its three interior angles are equal to two right angles.
— Ethics, Part 1, XVII
God was truly an infinite being that necessitated all truths both material and mental. All truths could be deduced from God the same way that it could be deduced from mathematics that three interior angles of a triangle are equal to two right angles.
It’s a shame we don’t have all the complete works of the ancient Stoics but we know that they used a variety of methods to support their claims that the Universe was Divine and Providential. Zeno, for instance, used the ontological argument discussed earlier. Zeno also used the argument from design which is particularly empirical. The Stoics weren’t afraid to use both a priori and a posteriori means that they had available at their disposal.
One thing the Stoics never did seem to use was the prime cause argument for the existence of God. This was primarily because it wasn’t popular among any of the Stoics to believe that God and Nature were separable. The Universe was the necessary being that necessitated all things throughout Itself.
One thing is for sure, Spinoza and the Stoics seemed to be on the same page that God is Nature and Nature is God. I’ll leave you with a quote from Marcus Aurelius:
Constantly regard the universe as one living being, having one substance and one soul; and observe how all things have reference to one perception, the perception of this one living being.