“Wanda dear, what is it that has you so engrossed?” asked Hari Seldon as he entered his granddaughter’s office at Streeling University. The room had been the office of the brilliant mathematician Yugo Amaryl, whose death had impoverished the Psychohistory Project. Fortunately, Wanda had gradually taken over Yugo’s role in recent years, further refining and adjusting the Prime Radiant.

“Why, I’m working on an equation in Section 33A2D17. See, I’ve recalibrated this section”—she gestured to a glowing violet patch suspended in midair in front of her face—“taking into consideration the standard quotient and— There! Just what I thought—I think.” She stepped back and rubbed her eyes.

“What is it, Wanda?” Hari moved in closer to study the equation. “Why, this looks like the Terminus equation and yet … Wanda, this is an inverse of the Terminus equation, isn’t it?”

“Yes, Grandpa. See, the numbers weren’t working quite right in the Terminus equation—look.” Wanda touched a contact in a recessed wallstrip and another patch sprang to life in vivid red on the other side of the room. Seldon and Wanda walked over to inspect it. “You see how it’s all hanging together fine now, Grandpa? It’s taken me weeks to get it this way.”

“How did you do it?” asked Hari, admiring the equation’s lines, its logic, its elegance.

“At first, I concentrated on it from over here only. I blocked out all else. In order to get Terminus to work, work on Terminus—stands to reason, doesn’t it? But then I realized that I couldn’t just introduce this equation into the Prime Radiant system and expect it to blend right in smoothly, as if nothing happened. A placement means a displacement somewhere else. A weight needs a counterweight.”

“I think the concept to which you are referring is what the ancients called ‘yin and yang.’ ”

“Yes, more or less. Yin and yang. So, you see, I realized that to perfect the yin of Terminus, I had to locate its yang. Which I did, over there.” She moved back to the violet patch, tucked away at the other edge of the Prime Radiant sphere. “And once I adjusted the figures here, the Terminus equation fell into place as well. Harmony!” Wanda looked pleased with herself, as if she’d solved all the problems of the Empire.

“Fascinating, Wanda, and later on you must tell me what you think it all means for the Project. —But right now you must come with me to the holoscreen. I received an urgent message from Santanni a few minutes ago. Your father wants us to call him immediately.”

Wanda’s smile faded. She had been alarmed at the recent reports of fighting on Santanni. As Imperial budget cutbacks went into effect, the citizens of the Outer Worlds suffered most. They had limited access to the richer, more populous Inner Worlds and it became more and more difficult to trade their worlds’ products for much needed imports. Imperial hyperships going in and out of Santanni were few and