XII. On the Measure of the Force of Testimony in Cases of Legal Evidence. By 

 John Tozer, Esq. M.A., Barrister-at-Law, Fellow of Gofiville and Cuius 



College. 



[Read Nov. 27, 1843.] 



* On the question of the possibility or advantage of measuring numerically the force of tes- 

 timony, the opinions which pervade the legal literature of the English language differ almost 

 invariably from the conclusions of science. This paper contains an attempt to trace the effect of 

 those conclusions in their application to a practical example, and to shew that they afford the 

 best means of analysing the processes which are necessarily adopted in such examples. The mere 

 purpose of rendering demonstrated truths more accessible, might seem to assign to the observations 

 which follow a place in professional rather than in scientific literature: it must however be remem- 

 bered, that practical men are concei'ned with practical rules, and with principles no further than 

 may be sufficient to render those rules intelligible. The occasional devotion of time to higher pur- 

 suits can scarely be regarded by them as other than treasonable to their personal interests; the 

 assertion of the supremacy of science over art they must for the most part leave to the culti- 

 vators of science. 



The proposition that a moral certainty is a mathematical probability whose numerical measure 

 lies between unity and some definite numerical fraction, puts in issue either directly or indirectly 

 every question that can be raised on the subject treated of in this paper, though the subject itself 

 is of a much more limited extent than the proposition. The vague way in which the processes 

 by which this proposition, and those which must stand or fall with it, can alone be established 

 or disproved, have been described by even the ablest of our legal authorities, removes every 

 feeling of diffidence in approaching the subject. Professor Starkie, in speaking of the mode of 

 estimating the weight of the united testimony of numbers, says, " If definite degrees of probability- 

 could be attached to the testimony of each witness, the resulting probability in favour of their united 

 testimony would be obtained not by the mere addition of the numbers expressing the several pro- 

 babilities, hut by a process of multiplication.'''' 1 Starkie, 3rd ed. SS*. And in a work there cited 

 occurs this passage: " On one side of the equation are mentally collected all tlie facts and circum- 

 stances witich fiave an ajfirmatiue value ; and on tlie ottier, all those which eittier lead to an 

 opposite infereyice, or tend to diminish the weiglit or to shew the non-relevancy of all or any 

 of the circumstances which have been put into the opposite scale. The value of each sepa- 

 rate portion of the evidence is separately estimated, and, as in algebraic addition, the opposite 

 quantities, positive and negative, are united, and the balance of probabilities is what remains 

 as the ground of human belief atid judgment.'''' Wills on Circumstantial Evidence, 14. 



Symbolical language has given expression to no processes of greater refinement and beauty than 

 those employed in the investigations of the theory of probabilities. No elaborate o)ies are required 

 in this particular application of its principles; but the expression, "a process of multiplication." 

 conveys to the mind no adequate idea of the simplest of them. Subjects which have'been deemed 

 worthy of their attention by Laplace and Poisson cannot be thus dismissed. 



"The notions of those who have supjio.sed that mere moral prob.ibilities or relations could 

 ever be re|)reseMted by numbers or space, and thus be subjected to arithmetical analysis, caniuit 

 but be regarded as visionary and chimerical." Starkie 571. 



