, and
then turn your eyes upon a dark background, and observe whether the
glowing filament appears there; this would be the "positive
after-image". This simple type of introspection is used by physiology
in its study of the senses, as well as by psychology; and it gives
such precise and regular results that only the most confirmed
behaviorists refuse to admit it as a good method of observation.

But psychology would like to make introspective observations on the
more complex mental processes as well; and it must be admitted that
here introspection becomes difficult. You cannot hope to make minute
observations on any process that lasts over a very few seconds, for
you must let the process run its natural course unimpeded by your
efforts at observing it, and then turn your "mental eye" instantly
back to observe it _retrospectively_ before it disappears. As a matter
of fact, a sensation or feeling or idea hangs on in consciousness for
a few seconds, and can be observed in this retrospective way. There is
no theoretical objection to this style of introspection, but it is
practically difficult and {11} tricky. Try it on a column of figures:
first add the column as usual, then immediately turn back and review
exactly what went through your mind in the process of adding---what
numbers you spoke internally, etc. Try again by introspecting the
process of filling in the blanks in the sentence:

"Botany could not make use of introspection because ______ have
probably no ________ processes."

At first, you may find it difficult to observe yourself in this way;
for the natural tendency, when you are aiming at a certain result, is
to reach the goal and then shift to something else, rather than to
turn back and review the steps by which you reached the goal. But with
practice, you acquire some skill in introspection.

One difficulty with introspection of the more complex mental processes
is that individuals vary more here than in the simpler processes, so
that different observers, observing each his own processes, will not
report the same facts, and one observer cannot serve as a check upon
another so easily as in the simpler introspection of after-images and
other sensations, or as in the observations made in other sciences.
Even well trained introspectionists are quite at variance when they
attempt a minute description of the thought processes, and it is
probable that this is asking too much of introspection. We mustn't
expect