evidence of the
senses.

Let us suppose, Fred mused, that deep within the mind there is some
matrix of thought that ties the human to this universe. A matrix that
could conceivably be altered, and when altered would automatically shift
the person to another universe that the altered matrix fitted.

The subconscious usually took time to absorb and react. That was another
thing his father had taught him to observe. Learn something, and it
takes from days to months for it to become lodged in the subconscious
and to rise into operation naturally from there.

John Henderson had taken six weeks to vanish after having learned the
theory. It had taken Horace Smith three and a half days, but he had had
the added factor of Dr. Henderson's disappearance to trigger reactions.
The theoretical physics class had taken three days exactly, and its
vanishing had been a sort of group action or chain reaction, with
intensely emotional reaction after the first student had vanished before
the eyes of the others.

His own father, originator of the theory, had probably fallen into the
trap of starting to believe after Horace had vanished, so it became a
greater probability that the disappearance was related to knowledge of
the theory. Seeing the students vanish had probably set up an emotional
state where complete belief was precipitated.

In the whole series the only improbable part was that so many students
would react in the same short time. That was partly nullified by the
fact that it was a special class, and only high I.Q. students with
excellent records were accepted. They would tend to be somewhat
identical in reaction times.

He straightened up and stared through the windshield at the dark street.
So there it was, the probable mechanism of vanishment. A system was fed
into the conscious mind. The conscious mind accepted it. In due time
that system was transferred down into the matrix that held the person in
this reality or universe. Once there, it made the whole person
_transfer_ to a system where the altered matrix fitted. It might not be
the system pictured in his father's theory. It might be a compromise
system.

_Where_ and _when_ probably had no meaning in relation to the two
systems. That was why, when the shift came, the person vanished
instantly without any strange manifestations of any kind.

Was it reversible? If so, then some of those who had vanished would
reappear eventually.

A sudden, startling thought made