"Well, look at it this way: suppose you have a wheel."
"All right," Forrester said obligingly. "I have a wheel."
"And this wheel has fifteen weights on it. They're spaced equally around
the rim, and the wheel's revolving at high speed."
Forrester kept his eyes closed. When he had the wheel nicely spinning,
he said: "Okay. Now what?"
"Well," Diana said, "as long as the weights stay in place, the wheel
spins evenly. But if you remove one of the weights, the wheel's out of
balance. It starts to wobble."
Forrester took one of the weights (Dionysus, a rather large, jolly
weight) off the wheel in his mind. It wobbled. "Right," he said.
"It can take the wobble for a little while. But unless the balance is
restored in time, the wheel will eventually break."
Hurriedly, Forrester put Dionysus back on the wheel. The wobble stopped.
"Oh," he said. "I see."
"Our power machine works in that sort of way. That is, it requires all
fifteen occupants. Dionysus has been dead for three years now, and
that's about the outside limit. Unless he's replaced soon, the machine
will be ruined."
Forrester opened his eyes. The wheel spun away and disappeared. "So you
found me to replace Dionysus. I had to look like him, so the mortals
wouldn't see any difference. And the psychological similarity--"
"That's right," Diana said. "It's the same as the wheel again. If you
remove a weight, you've got to put back a weight of the same magnitude.
Otherwise, the wheel's still out of balance."
"And since the power machine works through the nervous system--"
"The governing factor is that similarity. You've got to be of the same
magnitude as Dionysus. Of course, you don't have to be an _identical_
copy. The machine can be adjusted for _slight_ differences."
"I see," Forrester said. "And the fifteen power nodes--" Another idea
occurred to him. "Wait a minute. If there are only fifteen power nodes,
then how come there were so many different Gods and Goddesses among the
Greeks? There were a lot more than fifteen back then."
"Of course there were," Diana said, "but they weren't real Gods. As a
matter of fact, some of them didn't really exist."