The Khwarezmian polymath who measured the Earth's size with a single mountain and a careful angle, and who travelled into India in the entourage of a conqueror only to produce a calm, comparative study of Hindu religion, science, and custom that many modern scholars treat as an early model of cross-cultural inquiry — a thinker whose central discipline was distinguishing what he had verified from what he had merely been told.
When al-Biruni wanted to know how big the Earth was, he climbed a mountain in what is now Pakistan, measured the dip of the horizon, and worked the answer out from a single angle. His figure for the Earth's radius lands strikingly close to the modern value, though how close depends on which medieval unit you assume he used. The same man, travelling in the baggage train of a warlord who was sacking Indian cities, sat down to learn Sanskrit and write the most patient, least triumphant account of Hindu thought that any outsider had yet produced. He spent a lifetime insisting on one unfashionable rule: never confuse what you have checked with what you have only heard.