A person who doubts himself is like a man who would enlist in the ranks of his enemies and bear arms against himself. He makes his failure certain by himself being the first person to be convinced of it.

The saying frames self-doubt not as a neutral feeling but as an act of self-sabotage: to doubt oneself is to “enlist” with one’s enemies, supplying them with the very weapon they need—your own conviction that you will fail. The military metaphor emphasizes how internal attitudes can function like strategic choices in a conflict, determining outcomes before external forces do. Its significance lies in highlighting the performative power of belief: confidence does not guarantee success, but persistent self-disbelief can make failure more likely by weakening resolve, initiative, and endurance. The quote thus reads as a warning about the psychological preconditions of achievement.