Cudworth was the most significant philosopher among the Cambridge Platonists. Only a fraction of his manuscript writings on ethics and epistemology was published, and that posthumously: his Treatise concerning Eternal and Immutable Morality (1731) and his Treatise of Freewill (1838). The major work published in his lifetime was his The True Intellectual System of the Universe (1678), an antideterminist analysis of philosophical atheism in all its manifestations, both ancient and modern. The humanist learning in which this book is steeped masks the modernity of his philosophical outlook and especially, perhaps, his admiration for Descartes. For although Cudworth had major criticisms of Cartesianism, Descartes informs his philosophy in fundamental respects. He lauded Descartes as the reviver of ancient atomism. He regarded the Cartesian conception of body as inert extension as the most intelligible account of corporeality, and the premise of a theory of immaterial (i.e., spiritual) causation. He shared Descartes’ view of mind as a distinct substance from body. And he took clear and distinct perception as the principle of epistemological certainty. However, Cudworth was also critical of Descartes. As concerns dualism, the operative distinction for Cudworth is not between mind and body, but between force and matter, active and passive. Incorporeality is not coterminous with the mental – all self-moving substance is incorporeal; and there are mental activities that neither are conscious nor involve cogitation – in other words, “unconscious.” For Cudworth, the boundaries of the incorporeal and corporeal are softened by his conception of “plastic natures” that govern life processes and direct the workings of nature toward providential ends. He was critical of Descartes’ theory of animal mechanism and rejected his proof of the existence of God based on the truth of our faculties as circular (see Circle, Cartesian). And he criticized Descartes’ rejection of final causes. The fundamental error of Cartesianism was its misidentification of God's will as arbitrary indifference, not subject to divine wisdom. Such a conception of the deity destroys both moral and epistemological certainty, opening the way to irreligion, skepticism, atheism. In his Treatise concerning Eternal and Immutable Morality, he develops the ethical dimension of the argument and lays the basis for a theory of active mind. His theory of mind is developed to an elaborate degree in his A Treatise of Freewill and in his unpublished manuscript writings “On Liberty and Necessity.”

See also Animal; Atom; Cartesianism; Cause; Circle, Cartesian; Dualism; Substance