P5: The formal being of ideas admits God/Nature as a cause only insofar as he is considered as a thinking thing, and not insofar as he is explained by any other attribute. I.e., ideas, both of God/Nature’s attributes and of singular things, admit not the objects themselves, or the things perceived, as their efficient cause, but God/Nature himself, insofar as he is a thinking thing.

Dem.: This is evident from P3. For there we inferred that God/Nature can form the idea of his essence, and of all the things that follow necessarily from it, solely from the fact that God/Nature is a thinking thing, and not from the fact that he is the object of his own idea. So the formal being of ideas admits God/Nature as its cause insofar as he is a thinking thing. But another way of demonstrating this is the following. The formal being of ideas is a mode of thinking (as is known through itself), i.e. (by IP25 C), a mode that expresses, in a certain way, God/Nature’s nature insofar as he is a thinking thing. And so (by IP10) it involves the concept of no other attribute of God/Nature, and consequently (by IA4) is the effect of no other attribute than thought. And so the formal being of ideas admits God/Nature as its cause insofar as he is considered only as a thinking thing, etc., q.e.d.