Full Idea
Kant was the first to emphasize the distinction between understanding and reason in a definite way, establishing the finite and conditioned as the subject-matter of the former, and the infinite and unconditioned as that of the latter.
Gist of Idea
Kant showed that the understanding (unlike reason) concerns what is finite and conditioned
Source
report of Immanuel Kant (Critique of Pure Reason [1781]) by Georg W.F.Hegel - Logic (Encyclopedia I) §45 Add
Book Reference
Hegel,Georg W.F.: 'The Hegel Reader', ed/tr. Houlgate,Stephen [Blackwell 1998], p.157
A Reaction
This seems to match Plato's division of reality into the realm of experience and of the mind. I am inclined to see them as a unity, united by the many levels of abstraction. Frege is the modern spokesman for the Plato/Hegel view.