Full Idea
In the first part of our transcendental logic we defined the understanding as the faculty of rules; here we will distinguish reason from understanding by calling reason the faculty of principles.
Gist of Idea
Reason is distinct from understanding, and is the faculty of rules or principles
Source
Immanuel Kant (Critique of Pure Reason [1781], B356/A299)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.387
A Reaction
If we narrow the concept of rationality down to a concern with rules or principles, the concept of 'understanding' has to widen out to cover inferences from experience. Personally I think we can be rational about particulars as well as principles.