'To build a fire' refers to the essential act of creating a fire for warmth, survival, and comfort, particularly in harsh natural environments. This term highlights themes of human struggle against nature and the limitations of individual agency within a deterministic universe, emphasizing how environment and circumstance dictate one's fate and decisions. It serves as a pivotal element in illustrating the broader philosophical ideas associated with naturalism, where characters often find themselves at the mercy of their surroundings and internal instincts rather than exercising free will.