Full Idea
When the soul investigates by itself it passes into the realm of the pure, ever existing, immortal and unchanging, …and its experience is then what is called wisdom
Gist of Idea
The soul on its own enters a pure, unchanging and eternal realm, and experiences wisdom
Source
Plato (Phaedo [c.374 BCE], 79d)
Book Reference
Plato: 'Complete Works', ed/tr. Cooper,John M. [Hackett 1997], p.70
A Reaction
Plato is probably the main source of something we can call 'pure' reason. It is a bit surprising that it took 2,000 years before someone thought to attempt a critique of it.