Concerning the theory of Forms or Ideas in Plato – and by extension that of perception – we can find an interesting parallel in the account given by Shankara’s Advaita Vedanta. The Idea of ‘the Good’ (the supreme Idea in Platonic metaphysics) would correspond to the highest ontological principle in advaita: Atman-Brahma; in fact, there is only one ontological principle, one primary, or ultimate, reality, in both philosophers.* Objects of external perception are illusory in both accounts. In Plato they are only images (ikones) of Ideas, which alone are real, whereas in Shankara these objects are also just representations in the mind, phenomena, and described as ‘names and forms’. For example, ‘pot’ is just a name, its underlying ‘substance’ being only clay (this is only an analogy or illustration, since there is no talk of substance in Shankara’s philosophy). There are no objects, and no world, having a separate (objective!) existence, the only reality being Atman-Brahman.

* This makes of Plato a non-dualist, which is contrary to the commonly-held idea that he was a dualist, due to the emphasis given throughout the ages to his doctrine of the two Worlds, Intelligible and Phenomenal (or ‘World of Appearances’).

Tom McFarlane (writing in Quora): ‘What concepts really are is a philosophical question about their ontological status. The answer will depend upon the philosophy you adopt. A materialist would regard concepts as ultimately reducible to the functioning of the material brain, while an idealist would regard concepts (and matter) as ultimately reducible to mind or consciousness.

The most famous idealist account of concepts was given by Plato in his theory of forms. According to Plato, a concept (or form) is a general abstract pattern that has various sensory objects as specific instantiations. The concept of a tree, for example, is an abstract idea that makes it possible for us to experience trees, as such. Without the concept of a tree, we may have experiences of images but they are not recognized as being an experience of a tree without the concept of a tree. In Plato’s view, the concept of the tree is more fundamental than any particular tree because the concept is a necessary precondition for trees to exist as objects of experience. Plato called the experiences of trees as instantiations of the concept of a tree. For more discussion, see What is a form in Plato’s metaphysical theory?’