A failure is not always a mistake; it may simply be the best one can do under the circumstances. The real mistake is to stop trying.

Silly rabbit. CC BY 3.0 · Wikimedia Commons
B.F. Skinner
American psychologist who developed operant conditioning and radical behaviorism. Published Walden Two (1948) and taught at Harvard for decades.
2 quotes
Do not call a man a failure until he has stopped trying.