This is one of those phrases you never define, except with other undefined phrases. I think we all do this to some degree. Most people don’t go around quoting experts or even reading academic works. These ideas have been integrated into culture. It’s not really bad, until someone takes some words like that and makes a claim that doesn’t match with the reality of nature.
Anyway. I wasn’t asking you to read the whole linked page, just pointing to it as something that matched your thoughts. Spinoza’s classic is “Ethics”. To say why we should act certain ways, he starts with the “substance” of nature. This is probably why people say he is pagan or pantheist. He’s also very logical, labeling his points and referring back to them with numbered propositions and definitions.
So, you can find things, like this study guide references:
Spinoza argues that there can be only one substance—it consists “of infinite attributes, each of which expresses eternal and infinite essence”—which he calls God. (Props I.11 & I.14) (Of particular interest is his second proof of Prop I.11 in which he argues from the possibility of God’s existence to God’s necessary existence.)
I can’t this link to the PDF to work. Go here
Then search for “Study guide to Spinoza’s Ethics, Part 1”
The Stanford page on Spinoza puts it this way:
According to the traditional Judeo-Christian conception of divinity, God is a transcendent creator, a being who causes a world distinct from himself to come into being by creating it out of nothing. God produces that world by a spontaneous act of free will, and could just as easily have not created anything outside himself. By contrast, Spinoza’s God is the cause of all things because all things follow causally and necessarily from the divine nature. Or, as he puts it, from God’s infinite power or nature “all things have necessarily flowed, or always followed, by the same necessity and in the same way as from the nature of a triangle it follows, from eternity and to eternity, that its three angles are equal to two right angles” (Ip17s1). The existence of the world is, thus, mathematically necessary. It is impossible that God should exist but not the world. This does not mean that God does not cause the world to come into being freely, since nothing outside of God constrains him to bring it into existence. But Spinoza does deny that God creates the world by some arbitrary and undetermined act of free will. God could not have done otherwise. There are no alternatives to the actual world—no other possible worlds—and there is no contingency or spontaneity within the world. Nothing could possibly have been otherwise. Everything is absolutely and necessarily determined.
There is a critique that Spinoza shifts at Proposition 15. As if he sees some flaws in his own line of reasoning, but keeps trying to hold it all together. He doesn’t really have an answer for why there is pain, or if people are just properties of God, so instead of “underlying substance” he talks of “sustaining cause” and all things “necessarily flowed” from God.
Section 2.1 of the Stanford page goes into this.
