I. -- Dialectic.

22. Existence and Nature of the Ideas. -- Dialectic -- the word is Plato's -- is the science of objective reality, and this latter is called the Idea (eidos, idea). Dialectic is therefore taken in the sense of metaphysics (and not in the more usual meaning, -- logic).

To establish the existence and the nature of those Ideas, Plato sets out with a fact of consciousness and with a postulate, both of which he takes from Socrates. The fact of consciousness is the presence in us of intellectual representations, whose object is both universal, necessary, and immutable. The postulate is the sincerity or validity of these mental representations, or, in other words, the thesis of dogmatic philosophy, that all or some of our mental representations have an extra-mental objectivity.

What is this reality which is the object of our conceptions? Whatever it be, the sense-world cannot contain it, because everything there is contingent, particular, changing and unstable (here we see the influence of Heraclitus); while real being, as we conceive it, must be endowed with the attributes of necessity, universality, unity, and immutability (here we see the influence of Parmenides and Pythagoras).

Plato infers, accordingly, that the real exists above and beyond the sense-world: the Idea is absolutely stable and exists by itself (ontôs on, auta kath auta) its isolation (chôrista) does not permit of its being considered either as the subjective product of the human understanding or as an operation of the Divine understanding. This latter interpretation of Plato, put forward by the Neo-Platonic philosophers and taken up enthusiastically in the Middle Ages by all those who would fain see in the Platonic dialectic an adumbration of the exemplarism of St. Augustine, conflicts with the most formal declarations of the founder of the Academy, -- as indeed Aristotle had already pointed out. This exaggerated Realism which invests real being with the attributes of thought, and proceeds to mould the real world according to the character of our mental representations, is at once the guiding principle and the fundamental error of Plato's metaphysic of Ideas.