such a _pragmatic applied psychology as will deal with all states of
mind that might possibly be involved in the determination and judgment
of crime_. It is the aim of this book to present such a psychology.
``If we were gods,'' writes Plato in the Symposium, ``there would
be no philosophy''--and if our senses were truer and our sense
keener, we should need no psychology. As it is we must strive hard
to determine certainly how we see and think; we must understand
these processes according to valid laws organized into a system--
otherwise we remain the shuttlecocks of sense, misunderstanding and
accident. We must know how all of us,--we ourselves, witnesses,
experts, and accused, observe and perceive; we must know how
they think,--and how they demonstrate; we must take into
account how variously mankind infer and perceive, what mistakes
and illusions may ensue; how people recall and bear in mind; how
everything varies with age, sex, nature, and cultivation. We must
also see clearly what series of influences can prevail to change all
those things which would have been different under normal conditions.
Indeed, the largest place in this book will be given to the
witness and the judge himself, since we want in fact, from the first
to keep in mind the creation of material for our instruction; but the
psychology of the criminal must also receive consideration where-
ever the issue is not concerned with his so-called psychoses, but
with the validation of evidence.

Our method will be that fundamental to all psychological investigation,
and may be divided into three parts:[1]

1. The preparation of a review of psychological phenomena.

[1] P. Jessen: Versuch einer wissenschaftlichen Begrundung der Psychologie.