The traditional theories of cognition do not arise out of a direct examination of ‘the contents of consciousness’ but as a consequence of metaphysical presuppositions. Once the fatal distinction has been made between reality and appearance, corresponding cognitive faculties must inevitably be set up; sense which presents us with appearances must be distinguished from thought which penetrates to, or intuits, or constructs—on this point there can still be disagreement— reality itself. Thus the general structure of Cudworth's theory of knowledge can be predicted from what we have said about his philosophy of nature: Eternal and Immutable Morality, in which Cudworth's epistemology is mainly developed, is, in a way, a series of footnotes to The True Intellectual System of the Universe. Certain of the details, however, are intrinsically interesting and of considerable relevance to Cudworth's ethical theory.

Cudworth maintains, with Descartes, that ‘sense is confused thought’, it being ‘no pure and sincere cogitation of the soul alone’. But the Cartesian formula has to be reinterpreted, in a way which brings it very close to the ‘secret doctrine’ of the Theaetetus, and must be understood thus: sensation, unlike thought, is not pure self-activity, although equally it is not pure passivity. We do not create our sensations, nor are they merely given to us. Both the process of sensation and the sensible objects arise out of the interaction of the corporeal and the incorporeal, the corporeal exerting pressure upon us, the mind displaying its vital energy. The mind, Cudworth insists, is never purely passive: ‘Sense of itself is not a mere passion, but a passive perception of the soul, which hath something of vital energy in it, because it is a cogitation.’ At the same time, he is not prepared to accept the doctrine he ascribes to ‘some of the Platonists’, viz. that sensation is simply the form of active knowledge in which the soul becomes conscious of what is going on in the body. And like Berkeley, he feels bound to admit that the soul cannot choose what sensations it shall have; hence that they are not simply of its creation. The soul, then, has passive as well as active energies; if this were not so, Cudworth argues, it could never be ‘vitally united to any body, neither could there be any such thing as an animal or living creature’.