222 Induction. [CHAP. IX. 



When, however, these successive classes are not ready 

 marked out for us by nature, and thence arranged in easily 

 distinguishable groups, the process is more obviously arbi 

 trary. Suppose we were considering the chance of a man s 

 house being burnt down, with what collection of attributes 

 should we rest content in this instance ? Should we include 

 all kinds of buildings, or only dwelling-houses, or confine 

 ourselves to those where there is much wood, or those which 

 have stoves ? All these attributes, and a multitude of others 

 may be present, and, if so, they are all circumstances which 

 help to modify our judgment. We must be guided here by 

 the statistics which we happen to be able to obtain in 

 sufficient numbers. Here again, rough distinctions of this 

 kind are practically drawn in Insurance Offices, by dividing 

 risks into ordinary, hazardous, and extra-hazardous. We 

 examine our case, refer it to one or other of these classes, 

 and then form our judgment upon its prospects by the sta 

 tistics appropriate to its class. 



22. So much for what may be called the mild form in 

 which the ambiguity occurs ; but there is an aggravated form 

 in which it may show itself, and which at first sight seems 

 to place us in far greater perplexity. 



Suppose that the different classes mentioned above are 

 not included successively one within the other. We may 

 then be quite at a loss which of the statistical tables to 

 employ. Let us assume, for example, that nine out of ten 

 Englishmen are injured by residence in Madeira, but that 

 nine out of ten consumptive persons are benefited by such a 

 residence. These statistics, though fanciful, are conceivable 

 and perfectly compatible. John Smith is a consumptive 

 Englishman ; are we to recommend a visit to Madeira in his 

 case or not ? In other words, what inferences are we to 

 draw about the probability of his death ? Both of the sta- 



