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Full Idea
'Concept' is a notion tied, in the classical Fregean manner, to cognitive significance. Concepts are distinct if we can judge rationally of one, without the other. Concepts are constitutively and definitionally tied to rationality in this way.
Gist of Idea
Concepts are distinguished by roles in judgement, and are thus tied to rationality
Source
Christopher Peacocke (Truly Understood [2008], 2.2)
Book Ref
Peacocke,Christopher: 'Truly Understood' [OUP 2008], p.60
A Reaction
It seems to a bit optimistic to say, more or less, that thinking is impossible if it isn't rational. Rational beings have been selected for. As Quine nicely observed, duffers at induction have all been weeded out - but they may have existed, briefly.