Many would be wise if they did not think themselves wise.

[Serían sabios algunos si no creyesen que lo son.]

Baltasar Gracián y Morales (1601-1658) Spanish Jesuit priest, writer, philosopher

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 176 (1647) [tr. Jacobs (1892)]
    (Source)

(Source (Spanish)). Alternate translations:

Some would be wise, if they did not think themselves so.
[Flesher ed. (1685)]

Some would be wise if they did not believe themselves wise.
[tr. Fischer (1937)]

Some would be sages if they did not believe they were so already.
[tr. Maurer (1992)]