Plato’s Forms: eternal, unchanging, non-spatial, non-temporal. The Form of Circle isn’t located anywhere and doesn’t change. Particular round things are imperfect, decaying instances. The Form is more real than the particulars because it’s stable.
Christian theology / Augustine: God plus immortal souls. The Forms become Ideas in the mind of God.
Descartes: two substances. Res cogitans (mind, unextended) and res extensa (matter, extended). Minds literally take up no space; bodies have no thought. They’re different kinds of stuff.
Bishop Berkeley: there is no matter at all. Only minds and their ideas. The “tree” outside is a bundle of perceptions held in being by God’s perceiving. (subjective idealism)
Kant: phenomena (the world-as-experienced, structured by our categories) vs. noumena (the thing-in-itself, which exists but is unknowable in principle).
Hegel: the Absolute Idea, a self-developing logical structure that unfolds through nature and history and comes to know itself in human (and especially philosophical) consciousness.
Modern property dualism (Chalmers et al.): consciousness/qualia as a fundamental, non-physical property over and above the physical.
