14 THE PRINCIPLES OF SCIENCE. 



and the devices of sorcery and witchcraft often work their 

 own ends. A man dies on the day which he has always 

 regarded as his last, from his own fears of the day. An 

 incantation effects its purpose, because care is taken to 

 frighten the intended victim, by letting him know his 

 fate m . In all such cases the mental condition is the cause 

 of apparent coincidence. 



In a second class of cases, the event A may make our 

 perception of B follow, which would otherwise happen 

 without being perceived. Thus it was seriously believed 

 as the result of investigation that more comets appeared 

 in hot than cold summers. No account was taken of the 

 fact that hot summers would be comparatively cloudless, 

 and afford better opportunities for the discovery of 

 comets 11 . Here the disturbing condition is of a purely 

 external character. Certain ancient philosophers held 

 that the moon's rays were cold-producing, mistaking the 

 cold caused by radiation into space for an effect of the 

 moon, which becomes visible at the time when the absence 

 of clouds permits radiation to proceed. 



In a third class of cases, our perception of A may make 

 our perception of B follow. The event B may be con- 

 stantly happening, but our attention may not be drawn to 

 it except by our observing A. This case seems to be 

 illustrated by the fallacy of the moon's influence on clouds. 

 The origin of this fallacy is somewhat complicated. In 

 the first place, when the sky is densely clouded the 

 moon would not be visible at all ; it would be necessary 

 for us to see the full moon in order that our attention 

 should be strongly drawn to the fact, and this would 

 happen most often on those nights when the sky was 

 cloudless. Mr. W. Ellis , moreover, has ingeniously 



m Lubbock, ' Origin of Civilization,' p. 1 48. 



n De Morgan's 'Essay,' p. 123. 



' Philosophical Magazine,' 4th Series (1867), vol. xxxiv. p. 64. 



