  The historical answer to this question is in part that Plotinus assumed that he was following Plato who, in Timaeus (30c; cf. Philebus 22c), claimed that the Form of Intelligible Animal was eternally contemplated by an intellect called ‘the Demiurge’.  This contemplation Plotinus interpreted as cognitive identity, since if the Demiurge were contemplating something outside of itself, what would be inside of itself would be only an image or representation of eternal reality (see V 5) – and so, it would not actually know what it contemplates, as that is in itself. ‘Cognitive identity’ then means that when Intellect is thinking, it is thinking itself.  Further, Plotinus believed that Aristotle, in book 12 of his Metaphysics and in book 3 of his De Anima supported both the eternality of Intellect (in Aristotle represented as the Unmoved Mover) and the idea that cognitive identity characterized its operation.