8. Descartes, Spinoza and Leibniz¶
Core Argument¶
Descartes posed the relation of thought to being in a manner fundamentally distinct from the English empiricists. His scepticism was positive, a deliberate method to secure certain knowledge by doubting all received opinions until he reached an indubitable foundation: “I think, therefore I am.” This assertion, however, does not logically follow; he could only affirm that thought exists, not a separate ‘I’. From this, Descartes adopted a dualist position: matter possesses extension, thought possesses none, and the two are diametrically opposed. The soul inhabits the body mechanically, leaving the body a lifeless automaton. This created an irresolvable problem: if thought and matter are completely separate, they cannot interact. Descartes’ only recourse was divine intervention, and his disciple Geulinx admitted the coincidence of will and bodily action was mere chance. Despite this idealist trap, Descartes’ physics was materialist: he endowed matter with self-creative power and conceived mechanical motion as its life. His science advanced natural science in France, but his unresolved dualism remained the philosophy’s Achilles’ heel.
Spinoza, inspired by Descartes but critical of him, developed his philosophy in the context of the Dutch Republic’s relative freedom, defending freedom of thought against Calvinist orthodoxy. He broke with Cartesian dualism by arguing that body and mind are two attributes of a single Substance, which is infinite, eternal, and identical with nature. Though Spinoza called this Substance “God”, equating God with nature effectively abolished a separate deity, earning him accusations of atheism. His philosophy is the real point of departure for dialectical materialism: substituting ‘matter’ for ‘God’ yields a consistent materialist position. Marx and Engels admired Spinoza; Engels affirmed that thought and extension are attributes of one substance. Spinoza held that matter is its own cause, moves according to inherent laws, and can neither be created nor destroyed. Thought has no existence apart from matter; it is an attribute of matter organised in a certain way. He rejected free will, arguing that freedom consists in understanding necessity. All phenomena are transitory forms (modi) of the single Substance. This monism, though formally idealist, is open to materialist interpretation and has been substantially vindicated by modern science.
Leibniz’s theory of knowledge opposes Locke’s empiricism from an objective idealist standpoint. He is considered the father of German idealism and is known for the doctrine that this is “the best of all possible worlds”, meaning God chose one world from an infinite number of possibilities. This comforted the wealthy aristocrats he served, but philosophically the claim is tautological: the existing world is the only one, so it is the best. However, Leibniz’s dialectical philosophy, echoing Heraclitus and anticipating Hegel, does not defend an unchanging status quo; he states in Monadology 22 that each present state follows from its preceding state, and in Monadology 23 that all bodies are in perpetual flux like rivers. Feuerbach grasped the contradiction in Descartes, Spinoza and Leibniz: they are idealists in general but materialists in particular. Oizerman argues that their rationalist systems, despite theological assumptions, were tied to positive sciences. This connection existed only because the sciences had not yet separated from philosophy. Once that separation was complete, metaphysical systems claiming knowledge of super-physical reality conflicted with scientific development focused on terrestrial problems. These systems were discredited and vanquished by French eighteenth-century materialism.