Stupidity lies in wanting to draw conclusions.

[L’ineptie consiste à vouloir conclure. […] Oui, la bêtise consiste à vouloir conclure.]

Gustave Flaubert (1821-1880) French writer, novelist

Letter to Louis Bouilhet (4 Sep 1850)
    (Source)

The phrase is used twice in the letter. The initial phrase is usually translated to "foolishness" or "folly," the second to "stupidity."