Full Idea
Pure reason is concerned with nothing but itself, and it can have no other concern, because what is given to it is not objects to be unified for the concept of experience, but cognitions of understanding to be unified for the concept of reason.
Gist of Idea
Pure reason is only concerned with itself because it deals with understandings, not objects
Source
Immanuel Kant (Critique of Pure Reason [1781], B708/A680)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.610
A Reaction
It is hard to accept this sharp division between 'understanding', which gets involved in experience, and this very "pure" reason, which seems in danger of solipsism, and is playing a private game. I think purity comes in degrees.