This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Descartes needed two substances, extended body and thinking mind, and spent real effort explaining how they interact without ever fully closing the gap. Spinoza's answer, a generation later, was to deny there are two substances to begin with, or even a plurality of finite substances at all. On Spinoza's system there is exactly one substance, which he calls God or Nature interchangeably, infinite, self caused, and expressed under infinitely many attributes, of which thought and extension are simply the two human beings happen to be able to perceive. A mind and its body, on this view, are not two interacting things but one and the same mode of the single substance, considered under two different attributes; the puzzle of how a mental event causes a physical one dissolves rather than getting solved, because there were never two separate things to connect in the first place. This is Rationalism pushed to its most demanding conclusion: not merely that reason can establish some truths independently of the senses, which Descartes had already argued, but that the entire structure of reality can in principle be derived geometrically, proposition by proposition, the way Euclid derives theorems from axioms, which is exactly the form Spinoza gave his own masterwork. Where Descartes left interaction as an acknowledged loose end, Spinoza's single substance has no loose end to explain, at the cost of a metaphysics far stranger and, to many of his contemporaries, far more alarming than anything in the Meditations. Reason, followed all the way, does not always arrive somewhere comfortable.