IN CASES OF LEGAL EVIDENCE. 145 



assured ourselves that to suspend our judgments from a fear of erring no more than three times out 



997 

 of 1000 would be to defeat the purposes for which laws were instituted ; measures a probability 



1000 ' ■' 



which we consider large enough to warrant decision ; and the testimony of this witness does therefore 

 warrant decision; and it would do no more if in our judgments the witness would be impelled to give 

 his evidence by any other cause than the presence of the acid no more than three times in a million ; 

 the actual value of the probability in such a case, perhaps cannot, and certainly need not, be assigned, 

 but the value of its inferior limit is definite, and its measure is a numerical fraction. The mind of 

 the person deciding may have done no more than perceive that the probability equalled or exceeded in 

 magnitude those on which he habitually decided in affairs of equal importance; but if called on to 

 assign measures to the probabilities he has employed, he must say that his decision would not be with- 

 held from a fear of erring three times in 1000, and that the chance of erring in the case before him 

 was within that limit : the employment of numbers is a consequence of the effort to be definite. If, 

 again, we wish to compare the effect of evidence on different minds, though each may say in a parti- 

 cular instance that enough has or has not been adduced to produce conviction, the answer to the 

 question, how much has been adduced ? or, how much will prod\ice conviction? is, and is necessarily, 

 a numerical fraction. The conclusiveness or inconclusivcness of evidence is then altogether inde- 

 pendent of the definiteness or iiulcfiniteness of the probability it raises; the only condition necessary 

 to conclusiveness is, that that probability should be measured by a numerical fraction which e.\ceeds 

 some given definite magnitude. As regards criminal cases, the nature of the evidence does not admit 

 that demonstration can be obtained ; we cannot therefore ensure that out of some definite number of 

 persons punished one innocent person will not be punished as guilty; the only effect of making 

 the standard of conviction indefinite, is to make the number of cases indefinite in whicli the wrongful 

 decision has occurred; but it leaves us in doubt as to whether the injustice is increased or diminished. 



It is humiliating to intellectual pride to admit that our best exertions will not protect us from 

 inflicting wrong on others, but nothing can be gained by shrinking from measuring the extent of our 

 ability to do so. " Selon Condorcet, la chance d'etre condamne injustement pourrait etre equiva- 

 lente a celle d^in danger que nous jugeons assez petite pour ne pas menie chercher a nous y 

 soustraire dans les habitudes de la vie ; car, dit il, la societe a bien le droit, pour sa siirete, d'ex- 

 poser un de ses membres a un danger dont la chance lui est, pour ainsi dire, indifferente; mais cette 

 consideration est beaucoup trop subtile dans une question aussi grave. Laplace donne une definition, 

 bien plus propre a eclairer la question, de la chance d'erreur qu''on est force d''admettre dans les 

 jugements en matiere criminelle. Selon lui cette probabilite doit etre telle qu'il y ait plus de 

 danger pour la surete publique, a Tacquittement d'un coupable, que de crainte de la condamnation 

 d'un innocent." Poisson sur la Prohahilite des Jugements. 5. 



Condorcet assumes that a man has no more fear of dying at 25 than at 20, and that he therefore 



neglects a probability measured by , and infers that we may neglect this in our decisions. 



Condorcet, Probabilite des Decisions. 



If in the term "danger to the public" we include the danger arising from a callousness or indif- 

 ference to the infliction of wrong, or from a diminution of respect for the laws, the definition of 

 Laplace seems unexceptionable. 



In the example taken below, the formula first obtained applies to all facts the truth of which 

 may be estaljlished or disproved by ex])eriment; it assumes that the witnesses giving their testimony 

 have no wish to deceive. The peculiarity in facts of this nature is, that the repeating and varying 

 of the experiments tends successively to eliminate the several causes by which the ap|)earances could 

 have been produced, and to leave the" fact attested as the only known cause by which they can he 

 accounted for. If the tribunal be competent to judge of the skill and success with wiiich the 

 experiments have been conducted, their detail is submitted to its consideration ; if it be not, the 

 concluMuns are partly arrived at by the witnesses themselves, and taken on trust by tlie tribunal. 



