SECT. 15.] Inverse Probability. 185 



to make our postulate, we are often left without any very 

 definite or rational guidance. 



15. When, however, we take what may be called, by 

 comparison with the above purely artificial examples, instances 

 presented by nature, much of this uncertainty will disappear, 

 and then all real distinction between direct and inverse 

 probability will often vanish. In such cases the causes are 

 mostly determined by tolerably definite rules, instead of 

 being a mere cloud-land of capricious guesses. We may 

 either find their relative frequency of occurrence by refer 

 ence to tables, or may be able to infer it by examination of 

 the circumstances under which they are brought about. 

 Almost any simple example would then serve to illustrate 

 the fact that under such circumstances the distinction 

 between direct and inverse probability disappears altogether, 

 or merely resolves itself into one of time, which, as will be 

 more fully shown in a future chapter, is entirely foreign to 

 our subject. 



It is not of course intended to imply that difficulties 

 similar to those mentioned above do not occasionally invade 

 us here also. As already mentioned, they are, if not inherent 

 in the subject, at any rate almost unavoidable in comparison 

 with the simpler and more direct procedure of determining 

 what is likely to follow from assigned conditions. What is 

 meant is that so long as we confine ourselves within the 

 comparatively regular and uniform field of natural sequences 

 and coexistences, statistics of causes may be just as readily 

 available as those of effects. There will not be much more 

 that is arbitrary in the one than in the other. But of course 

 this security is lost when, as will be almost immediately 

 noticed, what may be called metaphysical rather than na 

 tural causes are introduced into the enquiry. 



For instance, it is known that in London about 20 people 



