Full Idea
Reason, in a creature, is a faculty which enables that creature to extend far beyond the limits of natural instinct the rules and intentions it follows in using its various powers, and the range of its project is unbounded.
Gist of Idea
Reason enables the unbounded extension of our rules and intentions
Source
Immanuel Kant (Idea for a Universal History [1784], 2nd)
Book Reference
Kant,Immanuel: 'Political Writings', ed/tr. Reiss,Hans [CUP 1996], p.42
A Reaction
I'm inclined to identify the mind's creation of universals as the source of this power, rather than reason. Generalisations are infinitely extensible. Cantor's infinities are a nice example. Can't ideas be extended irrationally?