You cannot change what you are, only what you do.

Taken at face value, the line draws a sharp boundary between essence and action: whatever one’s innate nature, past, or temperament may be, the sphere of moral responsibility lies in choices and conduct. It can be read as a corrective to self-justifying fatalism (“that’s just who I am”) while also resisting the fantasy of total self-reinvention. In Pullman’s fiction, where characters are tested by loyalty, courage, and conscience, such a sentiment would underscore an ethics of agency—identity is revealed and judged through deeds rather than declarations. The aphorism thus emphasizes accountability and the possibility of improvement through behavior, even when inner impulses remain stubborn.