Spinoza begins by assuming that, because substance is ultimate and totally independent, and because substances cannot interact, there can be only one substance.

Spinoza

Baruch Spinoza, a philosopher of Portugese-Jewish origin, who proposed that God and Nature were two names for the same reality, leading to his excommunication from the Jewish community.

Substance

The essential reality of a thing or things that underlies the various properties and changes of properties. Its most common definitions: “that which is independent and can exist by itself”; “the essence of a thing which does not and cannot change.” In traditional metaphysics, substance is the same as “ultimate reality,” and the study of substance is that branch of metaphysics that studies reality—namely, ontology.

Independent

Describes a state or condition of being free from outside control or not depending on something else.