Literally, criminal psychology should be _that form of psychology used in dealing with crime_; not merely, the psychopathology of criminals, the natural history of the criminal mind. But taken even literally, this is not all the psychology required by the criminalist. No doubt crime is an objective thing. Cain would actually have slaughtered Abel even if at the time Adam and Eve were already dead. But for us each crime exists only as we perceive it,--as we learn to know it through all those media established for us in criminal procedure. But these media are based upon sense-perception, upon the perception of the judge and his assistants, i.e.: upon witnesses, accused, and experts. Such perceptions must be psychologically validated. The knowledge of the principles of this validation demands again a special department of general psychology--even 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:[33] 1. The preparation of a review of psychological phenomena. 2. Study of causal relationships. 3. Establishment of the principles of psychic activity. The subject-matter will be drawn on the one hand, from that already presented by psychological science, but will be treated throughout from the point of view of the criminal judge, and prepared for his purposes. On the other hand, the material will be drawn from these observations that alone the criminologist at work can make, and on this the principles of psychology will be brought to bear.