s aspects and have accounted for every little detail； we have explained it and have done our duty。
But the word explain does not lead us very far。 It is mainly a matter of reducing the mass of the inexplicable to a minimum and the whole to its simplest terms。 If only we succeed in this reduction！ In most cases we substitute for one well…known term； not another still better one； but a strange one which may mean different things to different people。 So again； we explain one event by means of another more difficult one。 It is unfortunate that we lawyers are more than all others inclined to make unnecessary explanations； because our criminal law has accustomed us to silly definitions which rarely bring us closer to the issue and which supply us only with a lot of words difficult to understand instead of easily comprehensible ones。 Hence we reach explanations both impossible and hard to make； explanations which we ourselves are often unwilling to believe。 And again we try to explain and to define events which otherwise would have been understood by everybody and which become doubtful and uncertain because of the attempt。 The matter becomes especially difficult when we feel ourselves unsure； or when we have discovered or expect contradiction。 Then we try to convince ourselves that we know something； although at the beginning we were clearly enough aware that we knew nothing。 We must not forget that our knowledge can attain only to ideas of things。 It consists alone in the perception of the relation and agreement； or in the incompatibility and contradiction of some of our ideas。 Our task lies exactly in the explication of these impressions； and the more thoroughly that is done the greater and more certain is the result。 But we must never trust our own impressions merely。 ‘‘When the theologian； who deals with the supersensible； has said all that； from his point of view； he can say； when the jurist； who represents those fundamental laws which are the result of social experience； has considered all reasons from his own point of view； the final authority in certain cases must be the physician who is engaged in studying the life of the body。''
I get this from Maudsley；＇1＇ and it leads us to keep in mind that our knowledge is very one…sided and limited； and that an event is known only when all have spoken who possess especial knowledge of its type。 Hence； every criminalist is required to found his knowledge upon that of the largest possible number of experts and not to judge or discuss any matter which requires especial information without having first consulted an expert with regard to it。 Only the sham knows everything； the trained man understands how little the mind of any individual may grasp； and how many must coperate in order to explain the very simplest things。
＇1＇ Henry Maudsley： Physiology and Pathology of the Mind。
The complexity of the matter lies in the essence of the concept ‘‘to be。'' We use the word ‘‘to be'' to indicate the intent of all perceived and perceivable。 ‘‘ ‘To be' and ‘to know' are identical in so far as they have identical content； and the content may be known？''＇1＇
＇1＇ Jessen： Versuch einer wissenschaftlichen Begrndung der Psychologie。 Berlin 1855。
PART II。
OBJECTIVE CONDITIONS OF CRIMINAL INVESTIGATION： THE MENTAL ACTIVITY OF THE EXAMINEE。
TITLE A。 GENERAL CONDITIONS。
Topic I。 OF SENSE…PERCEPTION。
Section 35。
Our conclusions depend upon perceptions made by ourselves and others。 And if the perceptions are good our judgments _*may_ be good； if they are bad our judgments _*must_ be bad。 Hence； to study the forms of sense…perception is to study the fundamental conditions of the administration of law； and the greater the attention thereto； the more certain is the administration。
It is not our intention to develop a theory of perception。 We have only to extract those conditions which concern important circumstances； criminologically considered； and from which we may see how we and those we examine； perceive matters。 A thorough and comprehensive study of this question can not be too much recommended。 Recent science has made much progress in this direction； and has discovered much of great importance for us。 To ignore this is to confine oneself merely to the superficial and external； and hence to the inconceivable and incomprehensible； to ignoring valuable material for superficial reasons； and what is worse； to identifying material as important which properly understood has no value whatever。
Section 36。 （a） General Considerations。
The criminalist studies the physiological psychology＇1＇ of the senses and their functions； in order to ascertain their nature； their influence upon images and concepts； their trustworthiness； their reliability and its conditions； and the relation of perception to the object。 The question applies equally to the judge； the jury； the witness； and the accused。 Once the essence of the function and relation of sense…perception is understood； its application in individual cases becomes easy。
＇1＇ For a general consideration of perception see James； Principles of Psychology。 Angell； Psychology。
The importance of sense…perception need not be demonstrated。 ‘‘If we ask；'' says Mittermaier； ‘‘for the reason of our conviction of the truth of facts even in very important matters； and the basis of every judgment concerning existence of facts； we find that the evidence of the senses is final and seems； therefore； the only true source of certainty。''
There has always； of course； been a quarrel as to the objectivity and reliability of sense…perception。 That the senses do not lie； ‘‘not because they are always correct； but because they do not judge；'' is a frequently quoted sentence of Kant's； the Cyrenaics have already suggested this in asserting that pleasure and pain alone are indubitable。 Aristotle narrows the veracity of sensation to its essential content； as does Epicurus。 Descartes； Locke and Leibnitz have suggested that no image may be called； as mere change of feeling； true or false。 Sensationalism in the work of Gassendi； Condillac； and Helvetius undertook for this reason the defense of the senses against the reproach of deceit； and as a rule did it by invoking the infallibility of the sense of touch against the reproach of the contradictions in the other senses。 Reid went back to Aristotle in distinguishing specific objects for each sense and in assuming the truth of each sense within its own field。
That these various theories can be adjusted is doubtful； even if； from a more conservative point of view； the subject may be treated quantitatively。 The modern quantification of psychology was begun by Herbart； who developed a mathematical system of psychology by introducing certain completely unempirical postulates concerning the nature of representation and by applying certain simple premises in all deductions concerning numerical extent。 Then came Fechner； who assumed the summation of stimuli。 And finally these views were determined and fixed by the much…discussed Weber's Law； according to which the intensity of the stimulus must increase in the proportion that the intensity of the sensation is to increase； i。 e。； if a stimulus of 20 units requires the addition of 3 before it can be perceived； a stimulus of 60 units would require the addition of 9。 This law； which is of immense importance to criminalists who are discussing the sense…perceptions of witnesses； has been thoroughly and conclusively dealt with by A。 Meinong。＇1＇
＇1＇ Meinong： ber die Bedeutung der Weberschen Gesetzes。 Hamburg and Leipzig； 1896。
‘‘Modern psychology takes qualities perceived externally to be in themselves subjective but capable of receiving objectivity through our relation to the outer world。。。。 The qualitative character of our sensory content produced by external stimuli depends primarily on the organization of our senses。 This is the fundamental law of perception； of modern psychology； variously expressed； but axiomatic in all physiological psychology。''＇1＇ In this direction Helmholtz＇2＇ has done pioneer work。 He treats particularly the problem of optics； and physiological optics is the study of perception by means of the sense of sight。 We see things in the external world through the medium of light which they direct upon our eyes。 The light strikes the retina； and causes a sensation。 The sensation brought to the brain by means of the optic nerve becomes the condition of the representation in consciousness of certain objects distributed in space。。。。 We make use of the sensation which the light stimulates in the mechanism of the optic nerve to construct representations concerning the existence； form； and condition of external objects。 Hence we call images perceptions of sight。 （Our sense…perception； according to this theory； consists； therefore； entirely of sensations； the latter constitute the stuff or the content from which the other is constructed）。 Our sensations are effects caused in our organs； externally； and the manifestation of such an effect depends essentially upon the nature of the apparatus which has been stimulated。
＇1＇ T。 Pesch Das Weltphnomen
＇2＇ H。 Helmholtz： Die Tatsachen der Wahrnehmung。 Braunsehweig 1878。
There are certain really known inferences； e。 g。； those made by the astronomer from the perspective pictures of the stars to their positions in space。 These inferences are founded upon well… studied knowledge of the principles of optics。 Such knowledge of optics is lacking in the ordinary function of seeing； nevertheless it is permissible to conceive the psychical function of ordinary perception as unconscious inferences； inasmuch as this name will completely distinguish them from the commonly so…called conscious inferences。
The last…named condition is of especial importance to us。 We need investigation to determine the laws of the influence of optical and acoustical knowledge upon perception。 That these laws are influential may be verified easily。 Whoever is ignorant； e。 g。； that a noise is reflected back considerably； will say that a wagon is turning from the side from which the noise comes； though if he knows the law； if he knows that fact； his answer would be reversed。 S