Profound ignorance makes a man dogmatical; he who knows nothing thinks he can teach others what he just now has learned himself.

[C’est la profonde ignorance qui inspire le ton dogmatique. Celui qui ne sait rien croit enseigner aux autres ce qu’il vient d’apprendre lui-même.]

Jean de La Bruyère (1645-1696) French essayist, moralist

The Characters [Les Caractères], ch. 5 “Of Society and Conversation [De la Société et de la Conversation],” § 76 (5.76) (1688) [tr. Van Laun (1885)]
    (Source)

(Source (French)). Alternate translations:

Profound Ignorance makes a Man dogmatick. If he knows nothing, he thinks he can teach others what he is to learn himself.
[Bullord ed. (1696)]

Profound Ignorance makes a Man dogmatick; he who knows nothing, thinks he can teach others what he just now has learn'd himself.
[Curll ed. (1713)]

A dogmatic tone is generally inspired by abysmal ignorance. The man who knows nothing thinks he is informing others of something which he has that moment learnt.
[tr. Stewart (1970)]