On the ontology of ideas

Plato, Aristotle, Saussure

Plato thought Ideas, or the Forms, existed in an ultimate reality of essential types whose immutable existence demoted the world of experience to a lower ontological status of of appearances. Aristotle rejected Ideas, and proposed hylomorphic substances (ousia, perhaps this could be translated as Substantives to avoid the confusion with ordinary and chemical substantives), that those beings that could bear predicates had inseparable matter and form.

Descartes was familiar with Aristotle's hylomorphism of Substantives, but felt that it only applied to res extensa (extensional things). He was searching to demarcate the domain of natural philosophy from the domain of theology, with Galileo's censure from the church posing an ethical and philosophical dilemma for him (but not for us). Descartes solution was a Substantive dualism, to accept hylomorphism for concrete things in spacetime (res extensa) but not for moral beings (res cogitans) like angels and souls. For the domain of moral beings, Descartes accepted the principles of theology, and used a Platonic ontology of pure ideas.

Descartes' Substantive Dualism is still with us, not that many philosophical thinkers actually advocate it, but it sets up some ontological distinctions that still bedevil us in dealing with the mind-body problem. Descartes linked the ontological distinction of Material-Substantives and Ideal-Substantives with the epistemological distinctions of empirical grounding of certainty and a rational grounding of certainty. On the issue of first principles, Descartes came down on the rationalist side, using a priori introspection to achieve certainty of self. This inaugurates an individualist stance in Western philosophy, and attributes to introspection powers that perhaps exceed what is justified by science or philosophy. Decartes put the Ideal part of the distinction of Material-Substantive and Ideal-the issue of Substantive, as the basis for a rationalist a priori certain knowledge, conceding this crucial territory to the authority of theology. It was a strategic retreat from Galileo's theory-informed empiricism, that lead to the Copernican convictions that got him in trouble with the church. Descartes retreated to secure a large territory for empirical science, which flourished in the succeding centuries.

After Newton put celestial mechanics on a mathematical foundation, Galileo's Copernican outlook gained dominance with the New Physics that he inaugurated. Kant revisited issue of the ontological grounding of certain knowledge, and sided with Hume rather than Descartes. he introduced the additional distinction of synthetic vs. analytical judgments. A posteriori judgments were already the domain of science, but a priori judgments were still subject to old-style metaphysics. Kant's new critical metaphysics tried to recover the synthetic part of a priori judgments for science. (Analytic a priori judgments were already the domain of logic). Kant did this my limiting the powers of what was accessible to human knowledge in principle. We cannot know the thing-in-itself.