Abu Rayhan al-Biruni · Science
Al-Biruni’s scientific temper at full strength: his letters challenging Aristotle’s physics point by point against Ibn Sina, his treatment of the earth’s rotation as a legitimate testable question, and his exquisitely precise measurements of specific gravity - observation and experiment set above the word of authority.
While still a young man, al-Biruni did something audacious: he wrote to Ibn Sina - Avicenna, the towering philosopher-physician of the age - with a list of sharp objections to the physics of Aristotle, whose authority then stood almost beyond challenge. The exchange grew into a full correspondence, eighteen questions in all, ten of them aimed at Aristotle’s book On the Heavens and the rest at his physics, with Ibn Sina and later his pupil replying, and al-Biruni pressing back. What is striking is not that al-Biruni disagreed but how he disagreed: he kept asking whether Aristotle’s confident claims about the natural world actually matched observation and reason, rather than accepting them because Aristotle had said so. He questioned why the heavenly bodies should be denied all heaviness and lightness, why there could not be other worlds or a void beyond the cosmos, why celestial motion had to be perfectly circular. He was probing the foundations of the reigning physics with an empiricist’s persistence.
Nothing shows al-Biruni’s method better than his treatment of a daring idea: that the earth might rotate on its axis, producing the apparent daily turning of the heavens. Some Indian astronomers, such as Aryabhata, had entertained it, and al-Biruni took it seriously as a genuine, testable hypothesis rather than a heresy to be waved away. He made a crucial observation: as far as astronomy goes, the two pictures are equivalent - whether the heavens turn around a still earth or the earth turns beneath a still sky, the calculations and predictions come out the same. So astronomy alone cannot decide the question; it would have to be settled, if at all, on the grounds of physics. Al-Biruni himself did not finally adopt the rotating earth - he was troubled by physical objections of the day, such as why loose objects are not flung off or left behind - but the point is his stance: he framed a bold cosmological claim as an open empirical question, defined what evidence could and could not bear on it, and handed it to the physicist to resolve. That is scientific method in miniature.
Behind the letters, the rotating-earth question, and the density measurements lies a single conviction: that the final court of appeal in natural questions is observation and experiment, not the prestige of a name. Al-Biruni admired the Greeks and learned everything he could from Aristotle, Ptolemy, and Euclid - but he treated them as fallible predecessors to be checked, not oracles to be obeyed. Where he could test a claim, he tested it; where he could measure, he measured; where a question could not yet be settled, he said so honestly and marked what evidence would settle it. He criticised astronomers, including revered ones, when he judged that religious or philosophical prejudice had bent their science, and he applied the same standard to friend and authority alike. This is why al-Biruni is so often invoked as an early exemplar of the scientific spirit: not because he had a formal theory of method, but because he consistently practised the habit at its core - subordinating authority to evidence, and holding even his own conclusions open to correction by the world.
This is the opening of the lesson. The rest — the dialogue, the primary source, and the recall — is in the app.
You learned that al-Biruni pressed empirical objections against Aristotle in his correspondence with Ibn Sina, held that the earth’s rotation was a question astronomy alone could not settle (and left it to the physicist), and measured specific gravities with near-modern accuracy. Explain how these show experiment and…
Leads to Ibn Sina.
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