I get this from Maudsley,[170] 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 coöperate in order to explain the very simplest things. 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?”[171] 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[172] 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.