Ibn al-Haytham: The Man Who Invented the Scientific Method

Sometime around the year 1011, in Cairo, a scholar reached a conclusion that sounds obvious now and was radical then. If you want to know how vision works, do not reason about it from first principles in the Greek manner. Set up the conditions, watch what actually happens, and let the result overrule the theory, however distinguished the theorist. With that move, Ibn al-Haytham did something very close to inventing the modern habit of mind.

The man and the trap he set for himself

Ibn al-Haytham, Latinised as Alhazen, was born in Basra around 965 and made his name as an engineer. According to the traditional account, that reputation nearly destroyed him. He boasted that he could regulate the flooding of the Nile, and the Fatimid caliph al-Hakim, a ruler famous for his volatility, took him at his word and summoned him to Egypt to do it. Once Ibn al-Haytham saw the river and the scale of the problem, he understood that the technology of his age could not manage it. Failing al-Hakim was dangerous, so he is said to have feigned madness, which kept him under house arrest but alive until the caliph died. The confinement gave him years of forced quiet, and he used them to write.

The Book of Optics

His masterwork, the Kitab al-Manazir or Book of Optics, settled an argument the Greeks had left open. Euclid and Ptolemy had largely held that the eye sees by sending out rays. Ibn al-Haytham demonstrated the opposite: light travels from objects into the eye. He backed the claim with experiments using darkened rooms and small apertures, described the behaviour of the camera obscura with precision, and treated the geometry of light, reflection and refraction as mathematics.

Evidence over authority

The deeper revolution was his attitude. He wrote that the seeker after truth should make himself the enemy of everything he reads, testing each claim rather than deferring to it, precisely because human writers are prone to error. He turned that scepticism on Ptolemy, the greatest authority of the inherited tradition, and found him wanting. When his optics reached Europe in Latin, it shaped Roger Bacon, Witelo and eventually the men who built the new science of the seventeenth century. They were standing on a method that a prisoner in Cairo had laid out six hundred years earlier.

Beyond optics

Optics was only the most famous of nearly two hundred works attributed to him, on astronomy, geometry and number theory among other subjects. He analysed why the moon looks larger near the horizon, worked on problems that later mathematicians studying sums of powers would return to, and questioned the physical models behind Ptolemy's astronomy. A crater on the Moon is named Alhazen in his honour, a fitting address for a man who insisted on looking before believing.

The Ibn al-Haytham print belongs to the Riwayah Golden Age collection. You can see it here.

The method Ibn al-Haytham set down was one achievement of a much larger age. For the wider context, see what the Islamic Golden Age actually was.