"We only had a theory to work on," said Morrel, staring gloomily at the curved window. "A theory and an equation. The theory said that a man returning through time could alter the social and technological trends of the people and times to which he returned, in order to change history that was already past. The theory said that if we could turn the social patterns and technological trends just slightly away from what they were, we could alter the entire makeup of society in our own time. And the Equation was the tool, the final check on any change. The Equation which evaluates the sum of social, psychological and energy factors in any situation, any city or nation or human society. The Equation has been proven, checked time and time again, but the theory didn't fit it. The theory was wrong." Roger Strang sat up, suddenly alert. "That boy," he said, his voice sharp. "You nearly made a sieve of him, trying to shoot him. Why didn't he die?" "Because he was on a high-order variable. Picture it this way: From any point in time, the possible future occurrences could be seen as vectors, an infinite number of possible vectors. Every activity that makes an alteration, or has any broad effect on the future is a high-order variable, but many activities have no grave implications for future time, and could be considered unimportant, or low-order variables. If a man turns a corner and sees something that stimulates him into writing a world-shaking manifesto, the high-order variable would have started when he decided to turn the corner instead of going the other way. But if he took one way home instead of another, and nothing of importance occurred as a result of the decision, a low-order variable would be set up. "We found that the theory of alterations held quite well, for low-order variables. Wherever we appeared, whatever we did, we set up a definite friction in the normal time-stream, a distortion, like pulling a taut rubber band out. And we could produce changes--on low-order variables. But the elasticity of the distortion was so great as to warp the change back into the time-stream without causing any lasting alteration. When it came to high-order changes, _we simply couldn't make any_. We tried putting wrong data into the machines that were calculating specifications for the Barrier, and the false data went in, but the answers that came out were answers that _should_ have appeared with the _right_ data. We tried to commit a murder, to kill David Strang, and try as we would we couldn't do it. Because it would have altered a high-order variable, and they simply _wouldn't be altered_!" "But you, Morrel," Roger exclaimed. "How about you? You were top man in the Barrier Base Security office. You must have made an impression."