The first philosopher who taught this was Parmenides, who identified
Existence and Intelligence, and who does not place existence among
sense-objects, "for, thought is the same thing as existence."[246]
He adds[247] that existence is immovable, although being thought.
Parmenides thus denies all corporeal movement in existence, so as that
it might always remain the same. Further, Parmenides[248] compares
existence to a sphere, because it contains everything, drawing thought
not from without, but from within itself. When Parmenides, in his
writings, mentions the One, he means the cause, as if he recognized
that this unity (of the intelligible being) implied manifoldness.
In the dialogue of Plato he speaks with greater accuracy, and
distinguishes three principles: the First, the absolute One; the
second, the manifold one; the third, the one and the manifold. He
therefore, as we do, reaches three natures.