"Possible," said Kemal. "But it depends on so many suppositions."

"Any scenario we think of will depend on suppositions," said Tagiri. "But this one has one unique virtue."

"And what is that?" asked Kemal.

"This one would have created a future terrible enough for the Interveners to think it worthwhile to go back and erase their own time in order to eliminate the source of the disaster. Think of what it would have meant to human history, if the powerful, technology-wielding civilization that swept to dominance over the whole world was one that believed in human sacrifice. If Mesoamerican cults of torture and slaughter had come to India and China and Africa and Persia armed with rifles and linked by railroads."

"And tied together with a single, unified, powerful, and efficient bureacracy, the way the Romans were," added Diko. "The internal dissensions of Europe went a long way toward making their overlordship weaker and more tolerable."

Tagiri went on. "It's not hard to imagine that the Interveners, looking back, saw the Tlaxcalan conquest of Europe as the worst, most terrible disaster in the history of humanity. And then they saw Columbus's drive and ambition and personal charisma as the tool they could use to put a stop to it."

"What does this mean, then?" said Hassan. "Do we abandon our entire project, because stopping Columbus would be worse than what he and those who came after him actually caused in our history?"

"Worse?" asked Tagiri. "Who is to say which is worse? What do you say, Kemal?"

Kemal looked triumphant. "I say that if Hunahpu is right, which we can't prove, though he makes a good case, we learn only one thing: Meddling with the past is useless because, as the Interveners proved, the mess you make is little better than the mess you avoid."

"Not so," said Hunahpu.

Everyone turned to look at him, and he realized that, caught up in the discussion, he had forgotten whom he was dealing with -- that he was contradicting Kemal, and in front of Tagiri and Hassan, no less. He glanced over at Diko, and saw that, far from looking worried, she simply gazed at him with interest, waiting to hear what he would say. And he realized that this was how all of them were looking at him, except Kemal, and his scowl was probably not personal -- it seemed to be his permanent expression. For the first time Hunahpu realized that he was being treated as an equal here, and they were not offended or contemptuous at his daring to speak. His voice was as good as anyone else's. The sheer marvel of it was almost enough to silence him.

"Well?" asked Kemal.

"I think what we learn from this," said Hunahpu, "is not that you can't intervene effectively in the past. After all, the Interveners did prevent exactly what they set out to prevent. I've seen a lot more of Mesoamerican culture than any of you, and even though it's my own culture, my own people, anyway, I can promise you that a world ruled by the Tlaxcalans or the Mexica -- or even the Maya, for that matter -- would never have given rise to the democratic and tolerant and scientific values that eventually emerged from European culture, despite all its bloody-handed arrogance toward other people."

"You can't say that," said Kemal. "The Europeans sponsored slave trade, and then gradually repudiated it -- who's to say that the Tlaxcalans wouldn't have repudiated human sacrifice? The Europeans conquered in the name of kings and queens, and by five centuries later they had stripped those monarchs, where they survived at all, of every shred of power they once had wielded. The Tlaxcalans would have evolved as well."

"But outside the Americas, wherever the Europeans conquered, native culture survived," said Hunahpu. "Altered, yes, but still recognizably itself. I think the Tlaxcalan conquest would have been more like the Roman conquest, leaving behind little trace of the ancient Gallic or Iberian cultures."

"This is all irrelevant," said Tagiri. "We aren't choosing between the Interveners' history and our own. Whatever else we do, we can't restore their history and we wouldn't want to. Whichever one was worse, ours or theirs, both were certainly terrible."

"And both," said Hassan, "led to some version of Pastwatch, some future in which they were aware of their past and able to judge it."

"Yes," agreed Kemal, rather nastily, "they both led to a time when meddlers with too much leisure on their hands decided to go back and reform the past to coincide with the values of the present. The dead are dead; let's study them and learn from them."

"And help them if we can," said Tagiri, her voice thick with passion. "Kemal, all we learn from the Interveners is that what they did was not enough, not that it shouldn't have been attempted at all."

"Not enough!"

"They were thinking only of the history they wanted to avoid, not of the history they would create. We must do better."

"How can we?" asked Diko. "As soon as we act, as soon as we change something, we run the risk of removing ourselves from history. So we can make only one change, as they did."

"They could make only one change," said Tagiri, "because they sent a message. But what if we send a messenger?"

"Send a person?"

"We have found, by careful examination, what the technology of the Interveners was. They didn't just send a message from their own time, because as soon as they started sending it, they would have destroyed themselves and the very instrument that was sending the message. Instead they sent an object back in time. A holographic projector, with their entire message contained within it. They knew exactly where to place it and when to trigger it. We've found the machine. It worked perfectly, and then it released powerful acids that destroyed the circuitry and, after about an hour, when no one was nearby, it released a burst of heat that melted itself into a lump of slag and then it exploded, scattering tiny molten fragments across several acres."

"You didn't tell us this," said Kemal.

"The team that is working on building a time machine has been aware of this for some time," said Tagiri. "They'll be publishing soon. What matters is this: They didn't just send a message, they sent an object. That was enough to change history, but not enough to shape it intelligently. We need to send back a messenger who can respond to circumstances, who can not only make one change but keep on introducing more changes. That way we can do more than simply avoid one dreadful path -- we can deliberately, carefully create a new path that will make the rest of history infmitely better. Think of us as physicians to the past. It isn't enough just to give the patient one injection, one pill. We must keep the patient under our care for an extended period, adapting our treatment to the course of the disease."

"You mean send someone into the past," said Kemal.

"One person, or several people," said Tagiri. "One person might get sick or have an accident or be killed. Sending several people would build some redundancy into our effort."

"Then I must be one of the ones you send," said Kemal.

"What!" cried Hassan. "You! The one who believes we should make no intervention at all!"

"I never said that, " said Kemal. "I only said that it was stupid to intervene when you had no way of controlling the consequences. If you are sending a team back into the past, I want to be one of them. So I can make sure it goes properly. So I can make sure it's worth doing."

"I think you have an inflated idea of your own powers of judgment," said Hassan crossly.

"Absolutely," said Kemal. "But I'll do it, all the same."

"If anyone goes at all," said Tagiri. "We need to go over Hunahpu's scenario and gather far more evidence. Then, whatever picture we emerge with, we must also plan what our changes will be. In the meantime we have scientists working on our machinebut working with confidence, because we've seen that a physical object can be pushed backward through time. When all these projects are complete -- when we have the power to travel back in time, when we know exactly what it is we're trying to accomplish, and when we know exactly how we intend to accomplish it -- then we'll make our report public and the decision whether to do it will be up to them. To everyone."