Kant points out that "the power of judgment is a special talent that cannot be taught but only practiced" and that the lack of such a power "cannot be made good by any school" (Critique of Pure Reason A133/B172; Guyer/Wood trans.). He then gives us this footnote:
The lack of the power of judgment is that which is properly called stupidity, and such a failing is not to be helped. A dull or limited head, which is lacking nothing but the appropriate degree of understanding and its proper concepts, may well be trained through instruction, even to the point of becoming learned. But since it would still usually lack the power of judgment (the secunda Petri), it is not at all uncommon to encounter very learned men who in the use of their science frequently give glimpses of that lack, which is never to be ameliorated.