Cavendish displayed a lifelong fascination with one of the hardest of the “hard” problems, the nature of infinity. In an age which saw the birth of calculus as well as revolutionary developments in cosmology, a consistent theory of infinity was generally regarded as an illusory goal. Cavendish tackled this vexing scientific problem, which represented a radical departure from the cosmological and theological consensus of the 1660s; it anticipates a new worldview which emerged toward the end of the century, in which biblical revelation was eventually subordinated to empirical science, the Copernican hypothesis triumphed over rival theories, and the notion of a plurality of worlds became commonplace rather than shocking. From the playful speculations of the 1650s, Cavendish’s confident analysis of the nature of infinity had evolved into an essential ingredient in her prescient “theory of everything.”