KiDD. — On Prohahility. . xlvii 



and again between this and the highest moral certainty, the snceessive 

 differences are merely differences of degree, while the shades of graduation 

 may be innumerable. We may refer for an example to the probable duration 

 of life. In the " English Life Tables," calculated by Dr. Farr from the census 

 of 1841, with other data, and published in conjunction with the Reports of the 

 Registrar-General, we find that of the men resident in England, and aged 30 

 years, one-half is, very nearly, the ratio of those who attain to the age of 66 

 years. This implies that in the case of an individual Englishman, aged 30, and 

 of whom we do not know that he is other than an average specimen, the 

 probability of his attaining to the age of QQ is equal to that of his dying 

 previously. That he will complete his 65th year is a little more likely j and 

 that he will complete his 67th year is, by a small difference, less probable. 

 But these three probabilities, or fractions of probability, are manifestly 

 homogeneous, the variation, in each instance, being merely a difference of 

 degree, and not a diversity of kind. And we might similarly go on, year by 

 year, through the still diminishing probabilities of survival, to the age of 90 or 

 100 j while on the other hand we might in like manner ascend with constantly 

 increasing likelihood, from the expectation that the individual in question 

 may survive the 66th year, till we should come to the year, the month, the 

 day, through which he is now passing. Although in such a series of proposi- 

 tions the lower grades are not probable, in the sense of being likely to be 

 realized, they have, however, each of them a fraction of probability. In the 

 absence of a term more appropriate, this word " probability " is therefore used 

 with an extended signification, so as to include all the grades of moral 

 certainty, likelihood, unlikelihood, and mere possibility. 



4. (Ambiguity.) — The important difference between the popular and the 

 scientific applications of the word " Probability " may occasion an injurious 

 ambiguity by ministering to one or other of the twin fallacies of credulity and 

 over-scepticism ; each of which is based upon ignoring the distinctions between 

 mere possibility and likelihood, or between lower probabilities and moral 

 certainty. That a speculation almost desperate may eventuate in success, is 

 possible ; there is some degree of probability, in the scientific sense, in favor of 

 the expectation : but such a result is improbable, unlikely ; and if important 

 interests are involved, to incur such a hazard is unwise. Again, the most 

 cogent circumstantial evidence may possibly mislead ; it is, therefore, only a . 

 conclusion of probability, in the generic application of the term, that A. B., 

 who is arraigned for murder upon such evidence, is guilty ; and, in cases of 

 this kind, it sometimes happens that, in despite of data affording a moral 

 certainty, the moral cowardice of jurors occasions a failure of justice. A like 

 fallacy has been incurred with reference to the tenet, that Mind does percep- 

 tibly preside over the operations and evolutions of Nature. I have heard 



