“One of the most interesting technical problems may or may not
be called psychology. The central problem of the mind, if you will, or the nervous
system, is this: when an animal learns something, it can do something different
than it could before, and its brain cell must have changed too, if it is made out of
atoms. In what way is it different ? We do not know where to look, or what to
look for, when something is memorized. We do not know what it means, or what
change there is in the nervous system, when a fact is learned. This is a very impor-
tant problem which has not been solved at all.”