Excerpt from The Golden Mean by Annabel Lyon, plus links to reviews, author biography & more

I can imagine the frosty incomprehension of my colleagues back in Athens. Science is the work of the mind, they will say, and here I am wasting my time swimming and grubbing.

"We cannot ascertain causes until we have facts," I say. "That above all must be understood. We must observe the world, you see? From the facts we move to the principles, not the other way round. "

"Tell me some more facts," the boy says.

"Octopuses lay as many eggs as poisonous spiders. There is no blood in the brain, and elsewhere in the body blood can only be contained in blood vessels. Bear cubs are born without articulation and their limbs must be licked into shape by their mothers. Some insects are generated by the dew, and some worms generate spontaneously in manure. There is a passage in your head from your ear to the roof of your mouth. Also, your windpipe enters your mouth quite close to the opening of the back of the nostrils. That's why when you drink too fast, the drink comes out your nose."

I wink, and the boy smiles faintly for the first time.

"I think you know more about some things than my tutor." The boy pauses, as though awaiting my response to this significant remark.

"Possibly," I say.

"My tutor, Leonidas."

I shrug as though the name means nothing to me. I wait for him to speak again, to help or make a nuisance of himself, but he darts back into the palace, just a boy running out of the rain.

Now here comes our guide, a grand-gutted flunky who leads us to a suite of rooms in the palace. He runs with sweat, even in this rain, and smiles with satisfaction when I offer him a chair and water. I think he is moulded from pure fat. He says he knows me, remembers me from my childhood. Maybe. When he drinks, his mouth leaves little crumbs on the inner lip of the cup, though we aren't eating.

"Oh, yes, I remember you," he says. "The doctor's boy. Very serious, very serious. Has he changed?" He winks at Pythias, who doesn't react. "And that's your son?"

He means Callisthenes. My cousin's son, I explain, whom I call nephew for simplicity; he travels with me as my apprentice.

Pythias and her maids withdraw to an inner room; my slaves I've sent to the stables. We're too many people for the rooms we've been allotted, and they'll be warm there. Out of sight, too. Slavery is known here but not common, and I don't want to appear ostentatious. We overlook a small courtyard with a blabbing fountain and some potted trees, almond and fig. My nephew has retreated there to the shelter of a colonnade, and is arguing some choice point or other with himself, his fine brows wrinkled and darkened like walnut meats by the knottiness of his thoughts. I hope he's working on the reality of numbers, a problem I'm lately interested in.

"You're back for the good times," the flunky says. "War, waah!" He beats his fat fists on his chest and laughs. "Come to help us rule the world?"

"It'll happen," I say. "It's our time."

The fat man laughs again, claps his hands. "Very good, doctor's son," he says. "You're a quick study. Say, 'I spit on Athens.'"

I spit, just to make him laugh again, to set off all that wobbling.

When he's gone, I look back to the courtyard.

"Go to him," Pythias says, passing behind me with her maids, lighting lamps against the coming darkness.

In other windows I can see lights, little prickings, and hear the voices of men and women returning to their rooms for the evening, public duties done. Palace life is the same everywhere. I was happy enough to get away from it for a time, though I know Hermias was disappointed when we left him. Powerful men never like you to leave.

"I'm fine here," Pythias says. "We'll see to the unpacking. Go."

"He hasn't been able to get away from us for ten days. He probably wants a break."

A soldier arrives to tell me the king will see me in the morning. Then a page comes with plates of food: fresh and dried fruit, small fish, and wine.

Excerpted from The Golden Mean by Annabel Lyon Copyright © 2010 by Annabel Lyon. Excerpted by permission of Knopf, a division of Random House, Inc. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.