1Two hundred years after the book came through the snow, the learning of Baghdad had spread to every city where Arabic was read. A man in Basra could read what a man in Córdoba had written the year before. And in Basra there lived a reckoner named al-Hasan, son of al-Haytham, who had read nearly everything and was sure of nearly all of it.
2He was a servant of God and a servant of the truth, and saw no quarrel between the two. But he was young and proud, and one day he said a thing he had not tested. He said: "If I were in Egypt, I could do something with its river. I could build a work that would hold back the flood in the wet years and let it go in the dry, so that the land never drowned and never starved."
3He had never seen the river. He had read about it.
4Words travel. These travelled to Cairo, to the ear of the caliph of Egypt, al-Hakim. He was a ruler whose favour came without warning, and whose anger came the same way, and men who served him made their wills first. The caliph sent gold to Basra, and a letter. The letter said: Come and do it.
5So Ibn al-Haytham came to Egypt, and he was afraid. The caliph gave him engineers and boats and he went up the river, week after week, into the south. And as he went he saw on either bank the works of the old Egyptians. He saw temples whose pillars ten men could not circle with their arms. He saw mountains of cut stone, laid so true that a knife would not go between the blocks.
6He looked at these, and a cold thought came to him, and it grew colder with every bend of the river. The people who built these could build anything. They lived by this river four thousand years. If the river could be held, they would have held it.
7He came at last to the place where the river breaks white over black rocks, and he stood on the bank with his measuring cords in his hands. He measured. He reckoned. He did it again. Then he sat down on a rock and put the cords away, for the answer was the one he had feared. It could not be done. Not with all the men and stone in Egypt.
8He had the long road north in which to think what he would say.
9He could have lied. He could have asked for more gold and more years, as others have done, and hoped the caliph would die first. Instead he went into the hall and stood before the most feared man in the world and said, "Commander of the Faithful, I was wrong. I spoke of a thing I had not seen. It cannot be done, and I am ashamed."
10There was a silence in which he heard his own blood. Then the caliph gave him, of all things, a post in the offices of government, and sent him away.
11But Ibn al-Haytham had seen the caliph's face. He knew that a mercy which comes without reason can leave the same way. He sat at his desk in the government office and waited each day for the hand on his shoulder. And at last he could bear it no longer, and he did the thing that this chapter is about.
12He went mad. He tore his papers. He spoke to people who were not there. He laughed in the wrong places and wept in the wrong places, until word of it was carried to the palace.
13Under the law, a madman could not be punished, for he did not know what he did. So the caliph's men took his goods, and shut him in a house, and set a guard on the door. And there he stayed. Not for a month. For ten years.
14Now there was a boy named Salim, the son of a washerwoman, who was twelve years old when the caliph's steward hired him to carry the madman's food.
15The steward was a careful man. He said to the boy, "You will take him bread and lentils twice a day and empty his pot. You will not speak to him. And listen. If ever you see him do a thing that a man in his right mind would do, if ever he speaks sense or reads or writes, you will come to me that same hour. There is a gold dinar in it for you."
16A gold dinar was a year of his mother's washing. Salim understood very well what was being bought. He also understood, though no one said it, what would happen to the madman afterward.
17For a month the madman sat in a corner and sang to the wall, and there was nothing to tell. But Salim was the kind of boy who looks. He saw that the madman's eyes, when he raved, were not on the boy but on the guard at the door behind him. He saw that the pot was always set neatly in the same place. He saw that the dust on the floor of the inner room had been swept.
18One noon he came early. The guard was asleep in the shade. The door of the inner room, which was always shut, stood a hand's breadth open, and Salim put his eye to the gap.
19The room inside was black. The shutters had been stopped with rags, so that no light came in at all, except in one place. In one shutter a hole had been bored, no wider than a lentil. And through it a thread of sunlight came in, perfectly straight, like a wire drawn tight, and made a small bright coin on the opposite wall.
20The madman stood beside it. He was holding a straight rod up against the beam, laying it along the light from the hole to the wall. He was not singing. His face was the stillest face Salim had ever seen.
21A board creaked. The man turned. For a long breath they looked at each other, the boy with the dinar in his eyes and the man with his death in the boy's mouth.
22Then the madman did not laugh, or throw anything, or sing. He said, in an ordinary quiet voice, "Come in and shut the door. You are letting in the light."
23Salim went in. He could not have said why. Perhaps because no grown man had ever needed him for anything before.
24"Look at the beam," said Ibn al-Haytham. "There is dust in the air and you can see its whole length. Tell me what shape it is. Do not tell me what you think I wish to hear. Tell me what you see."
25"It is straight," said Salim.
26"Good. Now I have seen it and you have seen it. That is twice as good as once." He rubbed his face. "For a year I have had no second pair of eyes. A thing that only one man has seen is not yet known. It is only believed."
27After that, Salim came early every day.
28One night the man set three lamps on a bench in the outer room, in a row, and took the boy into the dark room and shut the door. On the wall were three spots of light.
29"Which lamp makes which spot?" said Ibn al-Haytham.
30"The left lamp makes the left spot," said Salim.
31"Do you see that, or do you think it? Go out and cover the left lamp with the pot."
32Salim went and covered it. When he came back the spot that had gone dark was the one on the right.
33He stood there a while. Then he said slowly, "They cross. In the hole. Each light goes straight on its own road, and the roads cross in the hole, and none of them is spoiled by the others."
34The man gripped his shoulder so hard that it hurt. "Yes," he said. "Yes. A washerwoman's son, in one night."
35He told the boy then what he was hunting. The great ones of old, Euclid and Ptolemy, had taught that sight goes out of the eye. A person sees, they said, because the eye throws out rays that touch the world, as a blind man's stick touches a wall.
36"But look at the sun for a moment," said Ibn al-Haytham, "and it hurts you. Look at a bright window and shut your eyes, and the window is still there, burning. Does the hand hurt when it reaches out to touch? No. It hurts when it is struck. Something comes into the eye. Light comes off the lamp, and off the wall, and off your face, off every point of every thing, straight, in all directions. The eye is not a hand. The eye is a dark room with a small hole in it."
37Salim thought about this. "Then I do not go out to the world when I look," he said. "The world comes to me."
38"To you, and to me, and to the guard at the door, all alike, by the same roads. That is why we can ask each other what we see."
39In the third year the boy asked the question he had been carrying. It was night and they were waiting for the moon to reach the hole.
40"Master. You told the caliph the truth about the river, when it might have killed you. And then you told him a lie about your own mind. Why the one and not the other?"
41The man was quiet so long that Salim thought he had gone too far.
42"The first time, I was more ashamed than afraid," said Ibn al-Haytham. "The second time, I was more afraid than ashamed. That is all. I will not dress it up for you. A lie is a hole in a wall that lets in no light, and I made one, and I have lived inside it ever since. Look around you. This is what it bought me. One room." He looked at the bright thread from the shutter. "But I will tell you what I learned on the rock by the river. I had believed the books. I had not gone to see. A man who says a thing he has not tested is already lying a little, even if it happens to be true. I do not mean to do that again while I live."
43"And when you come out?"
44"If God wills that I come out, I will write down only what I have seen with my eyes and you have seen with yours, and I will write how we saw it, so that a man in another country can bore a hole in his own shutter and tell me I am wrong."
45The steward sent for Salim in the fifth year. He had grown suspicious. The boy stayed too long. He set two dinars on the table this time, and beside them a rod.
46"What does he do in there?"
47Salim was seventeen now, and he had learned something in the dark room about saying only what he had seen. He looked at the gold and at the rod. He thought of his mother. He thought of three spots of light on a wall.
48"He sits in the dark for hours and stares at a hole in the shutter," he said. "He makes me cover lamps with his chamber pot. He holds a stick up in the air and asks me if it is straight. Last week he had me carry a bowl of water into the dark and he put a rod in it and wept for joy because it looked broken."
49Every word of it was true. The steward stared at him. Then he swept the dinars back into his purse. "God protect us," he said. "He is worse than I thought." And he did not send for the boy again.
50Salim walked home shaking. He had told no lie. He had been taught not to. But he knew that he had laid true things side by side so that a false thing would be seen, and he did not feel clean. He told his master, and his master said, "No. Nor should you. Keep feeling it. I do."
51In the tenth year the caliph al-Hakim rode out alone one night into the hills east of the city, as his custom was, and he did not come back. His donkey was found, and his cloak. He was never seen again.
52When the news was sure, Ibn al-Haytham cut his hair and washed and walked out of his own front door past a guard who was no longer there. People in the street said it was a wonder, how his wits had come back to him. He did not argue.
53He was near sixty and owned nothing. He took a room by the great mosque of al-Azhar and earned his bread by copying out Euclid and Ptolemy, faithfully, every line. Then at night he wrote where they were wrong. He wrote his own great work on light and sight, in seven books, out of the ten years in the dark. There was a thing in it that no book of the Greeks had. He did not only say what was so. He said: do this, with this hole and this lamp, and see. Anyone who doubted him was handed the means to catch him.
54And he wrote another book, about the errors he had found in Ptolemy, whom he honoured. In it he set down what he had learned on the rock by the river. This is what he said in it.
55He wrote that the one who seeks the truth does not put his trust in the old books because they are old. He doubts them, and questions what he gathers from them. He "submits to argument and demonstration," and not to the word of any human being, for every human being is full of faults.
56Salim the washerwoman's son could read by then, for his master had taught him between the lamps. He became a maker of astrolabes in the street of the coppersmiths, and a good one. When apprentices brought him a thing they had heard, he would say what had been said to him: "Do you see that, or do you think it? Go and cover the lamp."
57As for the book on light, it went out by the roads that books went by. In Spain, long after, men of another faith put it into Latin. And in cold countries he had never heard of, people began to grind glass, and to ask the light where it had been.
58Now hear what came after.