The seventeenth-century philosopher Spinoza opposed the mathematician-philosopher René Descartes’s sharp division between mind and matter (so-called Cartesian dualism). Descartes claimed that there are two fundamental kinds of substance, material (or body, interpreted by Descartes as spatial extension) and mental (or thought). For Spinoza, in contrast, mind and matter, or thought and extension, are two aspects of a single underlying substance whose full nature we do not know, since it is infinite.

Spinoza emphasized the bodily parallel to all thought. He also made a deep and detailed analysis of the emotions, which are both bodily and mental. Spinoza has been portrayed as a forerunner of psychoanalytic theories of bodily expression of mental states and of psychosomatic illness, such as those of Freud and of other twentieth-century thinkers who emphasized the physiological and emotional bases of all thought. Spinoza was a thoroughgoing naturalist; that is, he denied that there is any aspect of reality that is not part of the natural world. Spinoza was a pantheist who identified God with nature (“God or nature”). He was also a monist who claimed not only that there is only one kind of substance, but also that there is numerically only one substance: God = universe.

Albert Einstein was a great admirer of Spinoza. Einstein admired Spinoza’s naturalism and sense of awe for and worship of the universe. Einstein, like Spinoza, did not believe in a personal God but had a religious awe for the mysteries of the cosmos. In some of Einstein’s more speculative interpretations of General Relativity there is only one thing: space-time. “Things” in the everyday sense (particles) are singularities or warps in space-time. This theory (later called geometrodynamics by John Wheeler) has a strong resemblance to Spinoza’s one substance view.

The monism of Spinoza and of geometrodynamics is an extreme form of holism. It claims not only that the whole is prior to the parts but that the parts do not have real existence at the most fundamental level; only the whole system does. Naess and some other ecologists support this form of holism and even relate it to geometrodynamics, as well as to Spinoza and pan-psychism (Mathews, 1991, 2003). The whole has properties that the parts do not possess. In many forms of holism, the properties of the whole cannot be fully explained simply on the basis of knowledge of the properties of the parts. Nevertheless, most biological holists are not monists. That is, the parts do have independent existence even if they are intimately related to one another. The genuine monist, such as Spinoza, denies that the parts have independent reality at all; they are simply “modes” or local modifications of the one real entity or substance.