404 THE PRINCIPLES OF SCIENCE. [CIIAP. 



what they are to look. It is only when excited and guided 

 by the hope of verifying a theory that the observer will 

 notice many of the most important points ; and, where the 

 work is not of a routine character, no assistant can super- 

 sede the mind-directed observations of the philosopher. 

 Thus the successful investigator must combine diverse 

 qualities ; he must have clear notions of the result he ex- 

 pects and confidence in the truth of his theories, and yet 

 he must have that candour and flexibility of mind which 

 enable him to accept unfavourable results and abandon 

 mistaken views. 



Instrumental and Sensual Conditions of Observation. 



In every observation one or more of the senses must be 

 employed, and we should ever bear in mind that the ex- 

 tent of our knowledge may be limited by the power of the 

 sense concerned. What we learn of the world only forms 

 the lower limit of what is to be learned, and, for all that 

 we can tell, the processes of nature may infinitely sur- 

 pass in variety and complexity those which are capable of 

 coming within our means of observation. In some cases 

 inference from observed phenomena may make us in- 

 directly aware of what cannot be directly felt, but we 

 can never be sure that we thus acquire any appreciable 

 fraction of the knowledge that might be acquired. 



It is a strange reflection that space may be filled with 

 dark wandering stars, whose existence could not have yet 

 become in any way known to us. The planets have 

 already cooled so far as to be no longer luminous, and it 

 may well be that other stellar bodies of various size have 

 fallen into the same condition. From the consideration, 

 indeed, of variable and extinguished stars, Laplace inferred 

 that there probably exist opaque bodies as great and 

 perhaps as numerous as those we see. 1 Some of these 

 dark stars might ultimately become known to us, either 

 by reflecting light, or more probably by their gravitating 

 effects upon luminous stars. Thus if one member of a 

 double star were dark, we could readily detect its exist- 

 ence, and even estimate its size, position, and motions, 



1 System of the World, translated by Harte, vol. ii. p. 335. 



