Ask a child how they see the trees outside the window, and most will tell you that light bounces off the leaves and goes into their eyes. Today that's a school-science answer. For more than a thousand years, though, many of the world's greatest thinkers believed the opposite.

The person who did the most to settle the question was a Muslim scholar named al-Hasan ibn al-Haytham. And one of the old printed copies of his great book sits right here in Massachusetts, in Harvard Yard.

Who was Ibn al-Haytham?

According to the University of Cambridge Digital Library, Ibn al-Haytham was born around 965 in Basra, in what is now Iraq. In 1020 he moved to Cairo, where he worked until his death around 1040. Europeans later called him "Alhacen" or "Alhazen," Latin versions of his first name, al-Hasan.

His most famous work is the Kitab al-Manazir, or Book of Optics. Cambridge says it is thought to have been written between 1028 and 1038. A Harvard Library exhibit text calls him "the father of modern optics" and describes the book as "a dramatic advance in the study of the behavior of light."

The discovery: light comes to the eye

Before Ibn al-Haytham, the leading view in Greek optics was that the eye sends out rays that reach toward objects. A review in a medical journal, archived by the U.S. National Library of Medicine, notes that Euclid and Ptolemy both held beliefs of this kind.

Ibn al-Haytham argued the other way. The review says an important observation in his Book of Optics led him to propose that "the eyes receive light reflected from objects, rather than emanating light themselves."

One simple argument helps explain why. If vision came from rays leaving the eye, looking at the sun should feel no different from looking at a candle. Instead, staring at the sun hurts. That makes sense if strong light is coming into the eye from outside.

What made his work last was more than the conclusion. It was the method. The same review says he combined observations with rational arguments, and that this approach had "a great influence" on later European scientists, including Roger Bacon and Johannes Kepler.

His book also traveled. Cambridge says the Arabic text was translated into Latin no later than 1210. Harvard historian of science Elaheh Kheirandish has noted that the work survives in five Arabic manuscripts but more than 20 in Latin.

The faith: "the seeker after truth"

Ibn al-Haytham is also remembered for how he described the search for knowledge. In a work called Doubts Concerning Ptolemy, he wrote these lines, in the translation quoted by UNESCO:

> "Truth is sought for its own sake ... A person who studies scientific books with a view to knowing the truth, ought to turn himself into a hostile critic of everything that he studies ... if he takes this course, the truth will be revealed to him."

That's a scientist's rule: don't accept a claim just because someone famous made it. Test it. And, as other translations of the same passage add, be suspicious of your own biases too.

Muslim readers may also hear a familiar echo in it. The Qur'an repeatedly invites people to look closely at the world. In Surah Ali 'Imran, it says (in the Quran.com translation):

> "Indeed, in the creation of the heavens and the earth and the alternation of the day and night there are signs for people of reason." (3:190)

The next verse describes people "who remember Allah while standing, sitting, and lying on their sides, and reflect on the creation of the heavens and the earth."

We should be careful here. The Qur'an doesn't teach the physics of the eye, and it would be wrong to claim it "predicted" Ibn al-Haytham's findings. His discovery came from observation, geometry and careful argument. What the verses offer is something different: a reason to look, and a spirit of humble reflection about what we find. Science answers how we see. Faith asks what it means that we can see at all. Many Muslims find room for both questions, and Ibn al-Haytham's life shows how one person could pursue both with seriousness.

The Massachusetts connection: a 1572 book in Harvard Yard

Harvard's Houghton Library, its rare book library, owns a printed Latin edition of Ibn al-Haytham's Optics: Opticae Thesaurus, published in Basel in 1572. According to Houghton's own records, it was bought in 1851 with a fund left by Horace A. Haven of Portsmouth, N.H., Harvard Class of 1842.

The book has been shown to the public several times. Harvard Library displayed it in a lobby case called "Islamic Science in Early Modern Europe" from Jan. 13 to May 1, 2025. That display also showed the first edition of Euclid's Elements printed in Arabic, with commentary by the Persian scientist Nasir al-Din al-Tusi. In 2008, a Houghton exhibit called "Windows into Early Science," organized by Kheirandish, included a reproduction of the 1572 book's title page. MIT students helped with that exhibit.

Kheirandish has written that this edition "represents an important chapter in the history of optics and perspective as well as experimental methods and conceptual breakthroughs."

Planning a visit

A few practical notes before you go:

  • It isn't on display right now. Houghton's current exhibitions listing doesn't include the Optics. Don't promise the kids they'll see it in a case.
  • The reading room is open to all. Houghton says its exhibits, events and reading room are open to everyone. Researchers must request materials before their visit, and appointments are recommended. Older students working on a history-of-science project can contact houghton_library@harvard.edu or call 617-495-2440 to ask about seeing the book.
  • Hours: On the day we checked, Houghton listed its exhibition galleries as open 10 a.m. to 5 p.m. and its reading room 10 a.m. to 4:45 p.m. Check the library's website before you go.
  • Access: The main entrance on Quincy Street has an automatic door and a ramp, and the exhibition spaces can be reached by elevator.

A question to take home

Next time the sun comes up over your town, ask your children: How do we know light comes into our eyes, and not out of them? Then tell them about a scholar from Basra who refused to accept an answer just because it was old. He said that truth "is sought for its own sake."

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