402 THE PRINCIPLES OF SCIENCE. [CHAP 



the density of gases from approximate vacuum upwards ; 

 we must subject liquids and solids to pressures or strains 

 of almost unlimited amount. 



Mental Conditions of Correct Observation. 



Every observation must in a certain sense be true, for 

 the observing and recording of an event is in itself an 

 event. But before we proceed to deal with the supposed 

 meaning of the record, and draw inferences concerning the 

 course of nature, we must take care to ascertain that the 

 character and feelings of the observer are not to a great 

 extent the phenomena recorded. The mind of man, as 

 Francis Bacon said, is like an uneven mirror, and does not 

 reflect the events of nature without distortion. We need 

 hardly take notice of intentionally false observations, nor 

 of mistakes arising from defective memory, deficient light, 

 and so forth. Even where the utmost fidelity and care 

 are used in observing and recording, tendencies to error 

 exist, and fallacious opinions arise in consequence. 



It is difficult to find persons who can with perfect fair- 

 ness register facts for and against their own peculiar views. 

 Among uncultivated observers the tendency to remark 

 favourable and forget unfavourable events is so great, that 

 no reliance can be placed upon their supposed observations. 

 Thus arises the enduring fallacy that the changes of the 

 weather coincide in some way with the changes of the 

 moon, although exact and impartial registers give no 

 countenance to the fact. The whole race of prophets and 

 quacks live on the overwhelming effect of one success, 

 compared with hundreds of failures which are unmen- 

 tioned and forgotten. As Bacon says, " Men mark when 

 they hit, and never mark when they miss." And we 

 should do well to bear in mind the ancient story, quoted 

 by Bacon, of one who in Pagan times was shown a temple 

 with a picture of all the persons who had been saved from 

 shipwreck, after paying their vows. When asked whether 

 he did not now acknowledge the power of the gods, 

 "Ay," he answered; "but where are they painted that 

 were drowned after their vows ? " 



If indeed we could estimate the amount of bias existing 

 in any particular observations, it might be treated like 



