Geometry and conjecture lead to the origin of physical extension. Calabi-Yau spaces, the modern-day correlates to Kant’s spheres, are approximated through quantum geometry. The units, superstrings (the branes of M-theory seen on edge), are smaller than the empirical threshold, the Planck length. Kant contended that the units are beyond the measure of the sensible. Modern cosmologists would agree, both in that investigating the absolutely large, the cosmos, necessarily leads to the investigation of the absolutely small, and in that this path leads from the empirical to the rational. The quest has become formal, and its physical pioneers are mathematicians. Beyond the scope of experiments, the explorers of strings and branes today do conceptual work. And like Kant, they stress the intelligible beauty of nature.