In the Critique of Pure Reason of 1781, Immanuel Kant separates knowing rules from applying them. The understanding, he writes, can be taught and equipped with rules. Judgment, however, is ein besonderes Talent …, welches gar nicht belehrt, sondern nur geübt sein will, a special talent that cannot be taught at all, only practiced.
A doctor, a judge or a statesman, Kant writes, can have many fine rules in his head and still easily go wrong in applying them. He may see the general in the abstract and not be able to tell whether a case in the concrete belongs under it. Or he has not been trained enough durch Beispiele und wirkliche Geschäfte, through examples and real business. Examples, Kant concludes, sharpen judgment.
In a footnote he adds: Der Mangel an Urteilskraft ist eigentlich das, was man Dummheit nennt. The lack of judgment is really what one calls stupidity.