An 11th-century book insisted you should test your theories, not just argue for them

Ibn al-Haytham (Alhazen), under house arrest in Cairo, wrote the Book of Optics, correctly explaining that vision works by light entering the eye rather than the eye emitting rays. More radically, he built experiments to check his own claims before trusting them, a habit historians now credit as an early articulation of the scientific method.

— Ibn al-Haytham, Book of Optics (Kitab al-Manazir) — c. 1021 CE