Nothing is built on stone; all is built on sand, but we must build as if the sand were stone.

The aphorism captures a characteristically Borgesian tension between metaphysical skepticism and ethical resolve. If “all is built on sand,” then human projects—knowledge, institutions, identities, even artworks—rest on contingency rather than unshakable foundations. Yet Borges’s imperative (“we must build as if the sand were stone”) refuses paralysis: it argues for acting, creating, and committing with the seriousness of certainty even when certainty is unavailable. The line thus frames life and art as disciplined fictions: we knowingly construct provisional structures, but we do so with craft, rigor, and responsibility, as though they could endure.