criminal psychology-第48章
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nd its justification is quite another question。
The above mentioned distinction may be made still clearer if instead of looking back to the form of the question； we study only the answer。 We have then to say that positive statements are justified by the existence of a single instance； negative assertions only by the complete enumeration of all possible instances and never at all if the instances be boundless。 That the negative proof always requires a series of demonstrations is well known； the one thing which may be firmly believed is the fact that the problem； whether a single example is sufficient； or a million are insufficient； is only a form of the problem of affirmative and negative assertions。
So then； if I ask： Has A ever stolen anything？ it is enough to record one judgment against him； or to bring one witness on the matter in order to establish that A committed theft at least once in his life。 If； however； it is to be proved that the man has never committed a theft； his whole life must be reviewed point by point； and it must be shown that at no instant of it did he commit larceny。 In such cases we are content with much less。 We say first of all： We will not inquire whether the man has never stolen。 We will see merely whether he was never punished for theft。 But here； too； we must beware and not commit ourselves to inquiring of all the authorities in the world； but only of a single authority； who； we assume； ought to know whether A was punished or not。 If we go still further； we say that inasmuch as we have not heard from any authorities that the man was ever punished for stealing； we suppose that the man was never punished on that ground； and inasmuch as we have not examined anybody who had seen A steal； we preferably suppose that he has never stolen。 This is what we call satisfactory evidence； and with the poor means at our disposal it must suffice。
In most cases we have to deal with mixed evidence； and frequently it has become habitual to change the problem to be solved according to our convenience； or at least to set aside some one thing。 Sup… pose that the issue deals with a discovered； well…retained footprint of a man。 We then suspect somebody and compare the sole of his shoe with the impression。 They fit in length and width； in the number of nails and in all the other possible indices； and we therefore assert： It is the footprint of the suspect； for ‘‘whose footprint？'' is the problem we are troubling ourselves to solve。 In truth we have only shown that the particular relations； in the matter of length； breadth； number of nails； etc。； agree； and hence we regard the positive part of the evidence as sufficient and neglect the whole troublesome negative part； which might establish the fact that at the time and in the region in question； nobody was or could be whose foot could accurately fit that particular footprint。 Therefore we have not proved but have only calculated the probability that at the time there might possibly not have been another person with a shoe of similar length； breadth and number of nails。 The probability becomes naturally less as fewer details come to hand。 The difficulty lies in finding where such probability； which stands for at least an assumption； must no longer be considered。 Suppose； now； that neither shoe…nails nor patches； nor other clear clews can be proved and only length and width agree。 If the agreement of the clews were really a substantiation of the proof by evidence； it would have to suffice as positive evidence； but as has been explained； the thing proved is not the point at issue； but another point。
The negative portion of the evidence will naturally be developed with less accuracy。 The proof is limited to the assertion that such shoes as were indicated in the evidence were very rarely or never worn in that region； also that no native could have been present that the form of the nails allowed inference of somebody from foreign regions； one of which might be the home of the suspect； etc。 Such an examination shows that what we call evidence is only probability or possibility。
Another form which seems to contradict the assertion that negative propositions are infinite is positive evidence in the shape of negation。 If we give an expert a stain to examine and ask him whether it is a blood stain； and he tells us： ‘‘It is not a blood stain；'' then this single scientifically established assertion proves that we do not have to deal with blood； and hence ‘‘negative'' proof seems brought in a single instance。 But as a matter of fact we deal here with an actually positive proof； for the expert has given us the deduced proposition； not the essential assertion。 He has found the stain to be a rust stain or a tobacco stain； and hence he may assert and deduce that it is not blood。 Even were he a skeptic； he would say； ‘‘We have not yet seen the blood of a mammal in which the characteristic signs for recognition were not present； and we have never yet recognized a body without the blood pertaining to it； and hence we may say； we are not dealing with blood because all of us found the characteristics of the stain to be what we have been until now accustomed to call the characteristics of rust stain。''
We have still to touch upon the difference between logical connection and experience。 If I say； ‘‘This mineral tastes salty； therefore it is soluble in water；'' the inference depends upon logical relationships； for my intent is： ‘‘If I perceive a salty taste； it has to be brought to the nerves of taste； which can be done only by the combination of the mineral with the saliva； hence by its solution in the saliva。 But if it is soluble in saliva it must also be soluble in water。'' If I say on the other hand； ‘‘This mineral tastes salty； has a hardness of 2； a specific gravity of 2。2； and consequently it crystallizes hexagonally；''this statement depends on experience； for what I really say is： ‘‘I know first of all； that a mineral which has the qualities mentioned must be rock salt； for at the least； we know of no mineral which has these qualities and is not rock salt； and which in the second place crystallizes hexagonally as rock salt does；a way which； at least； we find rock salt never to have missed。'' If we examine the matter still more closely we become convinced that in the first case only the formal and logical side； in the second the experiential aspect predominates。 The premises of both cases are purely matters of experience and the formal question of inference is a matter of logic。 Only；at one time the first question； at another the second comes more obviously into the foreground。 Although this matter appears self…evident it is not indifferent。 It is well known that whenever we are powerfully influenced by one thing； things of little intensity are either not experienced at all or only to a very small degree； and are therefore neglected。 This is a fact which may indeed be shown mathematically； for infinity plus one equals infinity。 When； therefore； we undergo great pain or great joy； any accompanying insignificant pain or any pleasure will be barely felt； just as the horses who drag a very heavy wagon will not notice whether the driver walking beside them adds his coat to the load （cf。 Weber's law）。 Hence； when we criminalists study a difficult case with regard to the question of proof； there are two things to do in order to test the premises for correctness accord… ing to the standards of our other experiences； and to draw logically correct inferences from these premises。 If it happens that there are especial difficulties in one direction while by some chance those in the other are easily removed； it becomes surprising how often the latter are entirely ignored。 And hence； the adjustment of inferences is naturally false even when the great difficulties of the first type are removed correctly。 Therefore； if the establishment of a fact costs a good deal of pains and means the expenditure of much time； the business of logical connection appears so comparatively easy that it is made swiftly andwrongly。
Mistakes become； at least according to my experience； still more frequent when the difficulty is logical and not empirical。 As a matter of honesty； let me say that
