To have doubted one’s own first principles is the mark of a civilized man.

Holmes’s remark links “civilization” not to certainty but to intellectual self-scrutiny. “First principles” are the deep assumptions—moral, political, religious, or legal—that people often treat as unquestionable. To have “doubted” them suggests a mature capacity for reflection: recognizing that one’s foundational beliefs may be contingent, inherited, or shaped by circumstance, and therefore subject to revision in light of experience and argument. In Holmes’s broader outlook (often skeptical, pragmatic, and anti-dogmatic), this kind of doubt is a safeguard against fanaticism and a precondition for tolerance. The line elevates humility and critical inquiry as civic virtues, implying that unexamined certainty is a mark of intellectual immaturity.