As always with Spinoza, it is helpful to begin with God. The system of ideas that are parallel to modes of extension constitutes God’s infinite intel… - Michael Della Rocca

How many things are there in the world? Spinoza’s answer: one.

According to Spinoza’s system, all reality flows with strict necessity from the nature of God. With the geometrical method, Spinoza captures the structure of this reality by deducing philosophical conclusions about reality from definitions which express the nature of God.

I think that one who appreciates the fact that, if there are to be genuine causal connections, they must amount to conceptual connections, is Hume. Hume, of course, denies that there are conceptual connections among distinct things and so he is unable to come up with genuine cases of causation. But, in a way, Hume does accept the rationalist demand that, if there is to be genuine causation, it must amount to conceptual connection. Spinoza accepts this rationalist demand too. But, unlike Hume, he sees there as being genuine conceptual connections, i.e. causal connections, in the world.