Reason is intelligence taking exercise. Imagination is intelligence with an erection.

The aphorism contrasts two faculties Hugo often pits against each other: disciplined rationality and the generative, world-making power of the imagination. “Reason” is framed as intelligence in a controlled, methodical mode—active, but bounded by rules and evidence. “Imagination,” by contrast, is depicted as intelligence in a heightened, aroused state: not merely thinking, but projecting, inventing, and reaching beyond the given. The deliberately provocative sexual metaphor underscores that imagination is not a separate faculty from intellect; it is intellect intensified—charged with desire, ambition, and creative force. The line also implies an aesthetic hierarchy: reason refines and tests, but imagination expands the possible and drives artistic creation.