In 1702, the German philosopher Leibniz coined the term idealism to characterize the metaphysical doctrine of Plato, an ancient philosopher whose work dates back to the first half of the 4th century BC. Its importance for idealism will make Whitehead say that “all Western philosophy is nothing more than a footnote added to the writings of Plato”. These take the form of different dialogues whose main protagonist, most often Socrates, is the spokesperson for Plato’s theses, and in particular, his theory of Ideas. This is never explicitly exposed by Plato but it is underpinned by a large part of his thought, notably in The Republic, the Phaedo, the Symposium and the Parmenides. The last Plato, more and more influenced by the Pythagorean thought, will tend to identify the Ideas and the Numbers, which he does not do in his older writings. In The Republic, the supreme Idea is the Good, in the sense of propriety, not of moral goodness. In the Symposium, the supreme Idea is the Beautiful.
Plato calls “Idea” or “Form” (translated from ἰδέα [idea] and εἶδος [eidos]) all intelligible realities, i.e. all conceivable and knowable things. From the impossibility of conceiving a definition and of knowing through the senses, he deduces that Ideas are the true objects of definition and knowledge. Ideas are immaterial and immutable, remaining eternally identical to themselves, universal when they manifest themselves in the sensible, alone really existing, and independent of thought. Unlike sensible appearances, which are contingent, inconsistent and changeable, Ideas are genuinely real. Plato emphasizes their reality by the addition of adjectives: “true” reality, for example, or by comparatives: “that which is most real”, in contrast with the sensible appearance which has no reality. only insofar as it possesses a certain relationship to the intelligible.
The opposition between the sensible and the intelligible consists first in Plato of an ontological separation between what is authentically real, the Idea, and what is not, the sensible appearance; to this strict separation corresponds an equally strict epistemological hierarchy: opinion concerns sensible appearances while science is knowledge of intelligible realities. The ontological opposition justifies an ontological (or metaphysical) idealism, while the division of knowledge justifies a form of epistemological idealism. The division of knowledge is expressed by Plato by means of the analogy of the line, which has both ontological and epistemological significance: the soul, in contact with a reality, is affected according to the nature of this reality. There will therefore be as many ways of being affected as there are modes of being, and these ways of being affected define ways of speaking about an object or of thinking about it.
Plato’s intelligible reality is often referred to as the “world of Ideas”. This expression is improper and comes from an overinterpretation of the dialogues by Philo of Alexandria. Plato speaks rather of the “sensitive place” and the “intelligible place” of the same world. The world, Plato explains in the Timaeus, is unique. His theory of Ideas is therefore not properly speaking a dualistic doctrine, opposing two realities of different types, but a monistic doctrine, accepting only one type of reality — the Idea. It is therefore in this sense an ontological idealism, sometimes called “objective” (following Hegel) because of its intellectualist character.
In Late Antiquity, following Plotinus in the 3rd century, the Neoplatonists identify the domain of Ideas with that of the Intellect, and the Supreme Form becomes the One, at the origin of the Intellect from which it emanates. The Intellect is the true being of things, while the One is beyond even being, as a “superessential” non-being from which everything originates. Contrary to this conception, and unlike classical Greek ontology, the Christian theology of late antiquity asserted itself by emphasizing the notion of person, understood as “interiority”, and the soul humanity is highlighted by its radical heterogeneity in relation to the world. Saint Augustine in particular, at the beginning of the 5th century, took as the starting point of his thought not the world, but the subject or consciousness. In this sense, he inaugurates a “subjective” form of idealism.
(Includes texts from Wikipedia with license CC BY-SA 3.0, translated and adapted by Nicolae Sfetcu)