Deniador said, "Very little. We can assume that there is a single planet on which the human species developed, because it is unlikely in the extreme that the same species, so nearly identical as to be interfertile, would develop on a number of worlds, or even on just two, independently. We can choose to call this world of origin Earth. The belief is general, here, that Earth exists in this corner of the Galaxy, for the worlds here are unusually old and it is likely that the first worlds to be settled were close to Earth rather than far from it."
"And has the Earth any unique characteristics aside from being the planet of origin?" asked Pelorat eagerly.
"Do you have something in mind?" said Deniador, with his quick smile.
"I'm thinking of its satellite, which some call the moon. That would be unusual, wouldn't it?"
"That's a leading question, Dr. Pelorat. You may be putting thoughts into my mind."
"I do not say what it is that would make the moon unusual."
"Its size, of course. Am I right? Yes, I see I am. All the legends of Earth speak of its vast array of living species and of its vast satellite-one that is some three thousand to three thousand five hundred kilometers in diameter. The vast array of life is easy to accept since it would naturally have come about through biological evolution, if what we know of the process is accurate. A giant satellite is more difficult to accept. No other inhabited world in the Galaxy has such a satellite. Large satellites are invariably associated with the uninhabited and uninhabitable gas-giants. As a Skeptic, then, I prefer not to accept the existence of the moon."
Pelorat said, "If Earth is unique in its possession of millions of species, might it not also be unique in its possession of a giant satellite? One uniqueness might imply the other."
Deniador smiled. "I don't see how the presence of millions of species on Earth could create a giant satellite out of nothing."
"But the other way around- Perhaps a giant satellite could help create the millions of species."
"I don't see how that could be either."
Trevize said, "What about the story of Earth's radioactivity?"
"That is universally told; universally believed."
"But," said Trevize, "Earth could not have been so radioactive as to preclude life in the billions of years when it supported life. How did it become radioactive? A nuclear war?"
"That is the most common opinion, Councilman Trevize."
"From the manner in which you say that, I gather you don't believe it."
"There is no evidence that such a war took place. Common belief, even universal belief, is not, in itself, evidence."
"What else might have happened?"
"There is no evidence that anything happened. The radioactivity might be as purely invented a legend as the large satellite."
Pelorat said, "What is the generally accepted story of Earth's history? I have, during my professional career, collected a large number of origin-legends, many of them involving a world called Earth, or some name very much like that. I have none from Comporellon, nothing beyond the vague mention of a Benbally who might have come from nowhere for all that Comporellian legends say."
"That's not surprising. We don't usually export our legends and I'm astonished you have found references even to Benbally. Superstition, again."
"But you are not superstitious and you would not hesitate to talk about it, would you?"
"That's correct," said the small historian, casting his eyes upward at Pelorat. "It would certainly add greatly, perhaps even dangerously, to my unpopularity if I did, but you three are leaving Comporellon soon and I take it you will never quote me as a source."
"You have our word of honor," said Pelorat quickly.
"Then here is a summary of what is supposed to have happened, shorn of any supernaturalism or moralizing. Earth existed as the sole world of human beings for an immeasurable period and then, about twenty to twenty-five thousand years ago, the human species developed interstellar travel by way of the hyperspatial Jump and colonized a group of planets.
"The Settlers on these planets made use of robots, which had first been devised on Earth before the days of hyperspatial travel and-do you know what robots are, by the way?"
"Yes," said Trevize. "We have been asked that more than once. We know what robots are."
"The Settlers, with a thoroughly roboticized society, developed a high technology and unusual longevity and despised their ancestral world. According to more dramatic versions of their story, they dominated and oppressed the ancestral world.
"Eventually, then, Earth sent out a new group of Settlers, among whom robots were forbidden. Of the new worlds, Comporellon was among the first. Our own patriots insist it was the first, but there is no evidence of that that a Skeptic can accept. The first group of Settlers died out, and-"
Trevize said, "Why did the first set die out, Dr. Deniador?"
"Why? Usually they are imagined by our romantics as having been punished for their crimes by He Who Punishes, though no one bothers to say why He waited so long. But one doesn't have to resort to fairy tales. It is easy to argue that a society that depends totally on robots becomes soft and decadent, dwindling and dying out of sheer boredom or, more subtly, by losing the will to live.
"The second wave of Settlers, without robots, lived on and took over the entire Galaxy, but Earth grew radioactive and slowly dropped out of sight. The reason usually given for this is that there were robots on Earth, too, since the first wave had encouraged that."