
Modern psychology takes completely for granted that behavior and neural function
are perfectly correlated, that one is completely caused by the other.  There is
no separate soul or lifeforce to stick a finger into the brain now and then and
make neural cells do what they would not otherwise.  Actually, of course, this
is a working assumption only....It is quite conceivable that someday the
assumption will have to be rejected.  But it is important also to see that we
have not reached that day yet: the working assumption is a necessary one and
there is no real evidence opposed to it.  Our failure to solve a problem so
far does not make it insoluble.  One cannot logically be a determinist in
physics and biology, and a mystic in psychology.
- D. O. Hebb, Organization of Behavior:  A Neuropsychological Theory, 1949
