In Book Three of Jalaluddin Rumi's Masnavi, Rumi uses the metaphor of bird flight to explain Sufi epistemology, contrasting certain knowledge ('ilm) with mere conjecture or opinion (zann). Rumi explains that conjecture is like a bird with only one wing: its flight is awkward, unsteady, and destined to fail. Certain knowledge, however, provides two wings, allowing the soul to soar straight and direct like Gabriel, completely unbothered by worldly opinions, doubt, or external validation.