Only one substance exists, and Spinoza calls it God or Nature

Spinoza's central metaphysical claim is that reality consists of a single, infinite substance that is entirely self-caused and self-sufficient, requiring nothing outside itself to exist or be explained. He identifies this substance with God, but redefines God radically: not a personal being separate from creation who chooses to make a world, but identical with the totality of nature itself, expressed through infinite attributes, of which humans can perceive only two — extension (physical matter) and thought (mind).

Everything we normally think of as individual things — rocks, trees, human bodies, minds — are not separate substances but finite "modes," or particular expressions, of this one underlying substance, the way waves are expressions of an ocean rather than independent entities. This collapses the traditional distinction between God and the natural world into a single, unified reality, a position historically labeled pantheism.

Takeaway: if everything is one substance expressing itself in different modes, the search for a separate divine cause behind nature dissolves.