For a long time, Plato's unwritten doctrines had been controversial. Many modern books on Plato seem to diminish its importance; nevertheless, the first important witness who mentions its existence is Aristotle, who in his "Physics" writes: "It is true, indeed, that the account he gives there in "Timaeus" of the participant is different from what he says in his so-called "unwritten teachings" ()." The term "" literally means "unwritten doctrines" or "unwritten dogmas" and it stands for the most fundamental metaphysical teaching of Plato, which he disclosed only orally, and some say only to his most trusted fellows, and which he may have kept secret from the public. The importance of the unwritten doctrines does not seem to have been seriously questioned before the 19th century.