394 DIVERSIONS OF A NATURALIST 



terranean water at a spot remote from the conduit, 

 where some large vats stood for the purpose of storing 

 rain-water ! All this, of course, tended to prove the 

 incompetence of the youth as a douser, and to make it 

 probable that such successes as he had obtained else- 

 where (and my hostess stated that they were very 

 numerous and remarkable, and vouched for by members 

 of her own family) were due to imposture. 



But a single case like this does not bring one very 

 far on the way to deciding the question as to whether 

 there are persons who are genuinely and successfully 

 guided to the discovery of subterranean water by strange 

 sensations and by spasmodic movements of their limbs or 

 of hazel-twigs held in the hands, due (as they declare) to an 

 obscure influence which emanates from subterranean water 

 and from buried metal. The fact is that we have in the 

 belief in the guidance of the douser by occult influences 

 a troublesome case of the fallacy in reasoning expressed 

 by the words, " post hoc ergo propter hoc," or, to put 

 it in English, "after this, therefore caused by this." 

 Primitive man found that this mode of forming a con- 

 clusion very often led to a correct discovery of the 

 connexion between two events, and he adopted it as a 

 ready method of guidance, although it was frequently 

 fallacious. It has taken ages, literally ages, to make 

 people discard this mode of arriving at a conclusion in 

 serious matters, and it is still usual in less vital affairs. 

 To show that B followed upon the occurrence of A, even 

 once, is, of course, a proper and useful way of forming a 

 guess or a suggestion as to the cause of B, but still more 

 is your guess legitimate if the sequence has occurred 

 several times in your experience. But it is only a 

 guess : a conclusion must not be accepted on that basis, 

 although lazy and hasty people do adopt such con- 



