We shall not espouse either pietism, scepticism, or criticism. We have merely to consider the individual phenomena, as they may concern the criminalist; to examine them and to establish whatever value the material may have for him; what portions may be of use to him in the interest of discovering the truth; and where the dangers may lurk that menace him. And just as we are aware that the comprehension of the fundamental concepts of the exact sciences is not to be derived from their methodology, so we must keep clearly in mind that the truth which we criminalists have to attain can not be constructed out of the _formal_ correctness of the content presented us. We are in duty bound to render it _materially_ correct. But that is to be achieved only if we are acquainted with principles of psychology, and know how to make them serve our purposes. For our problem, the oft-quoted epigram of Bailey’s, “The study of physiology is as repugnant to the psychologist as that of acoustics to the composer,” no longer holds. We are not poets, we are investigators. If we are to do our work properly, we must base it completely upon modern psycho physical fundamentals. Whoever expects unaided to find the right thing at the right moment is in the position of the individual who didn’t know whether he could play the violin because he had not yet tried. We must gather wisdom while we are not required to use it; when the time for use arrives, the time for harvest is over. Let this be our fundamental principle: _That we criminalists receive from our main source, the witnesses, many more inferences than observations_, and that this fact is the basis of so many mistakes in our work. Again and again we are taught, in the deposition of evidence, that only facts as plain sense-perceptions should be presented; that inference is the judge’s affair. But we only appear to obey this principle; actually, most of what we note as fact and sense-perception, is nothing but a more or less justified judgment, which though presented in the honestest belief, still offers no positive truth. “Amicus Plato, sed magis amica Veritas.”