Gratitude is the least of the virtues, but ingratitude is the worst of vices.

The epigram hinges on a paradox. Gratitude is called “the least of the virtues” not because it is unimportant, but because it is comparatively easy and baseline—an expected moral minimum in response to kindness. Yet its opposite, ingratitude, is “the worst of vices” because it negates relationship, memory, and justice: it treats generosity as meaningless and dissolves trust. Fuller implies that moral life is not only about grand acts of courage or piety; it is sustained by ordinary acknowledgments of dependence and benefit. The line also warns that ingratitude corrupts both giver and receiver by discouraging future benevolence and normalizing exploitation.