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."

Morrel smiled tiredly. "I really thought I had, time after time. I
would start off a series of circumstances that should have had a grave
alterative effect, and it would look for awhile as if a long-range
change was going to be affected--and then it would straighten itself
out again, with no important change occ