Self-correction begins with self-knowledge.

[Principio es de corregirse el conocerse]

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

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 69 (1647) [tr. Maurer (1992)]
    (Source)

(Source (Spanish)). Alternate translations:

The knowledge of one's self is the beginning of amendment.
[Flesher ed. (1685)]

Self-knowledge is the beginning of self-improvement.
[tr. Jacobs (1892)]

It is a first principle that in order to improve yourself, you must first know yourself.
[tr. Fischer (1937)]