PHILOSOPHICAL AND PSYCHOLOGICAL ASPECTS 241 



be held true by the next; because we designate as true that 

 which holds with the data available at a given time. By 

 this test, the Ptolemaic system in astronomy was truth for 

 the time being. Although later replaced by another concept, 

 Aristotle's explanation of respiration, as a means of cooling 

 the blood, was a good one for the knowledge of the period and 

 hence could be regarded as true in its day. Absolute truth 

 may be attainable, may, indeed, have been attained in some 

 instances. But when men believe that they have arrived at 

 finality in truth they get into trouble and when the scientist 

 thinks any truth is established for all time, science is in 

 danger of ossification. Scientists need to tell themselves 

 that their truths are working hypotheses and that truths 

 which seem firmly established may some day be overturned. 

 It is a fact that the simpler truths established by science 

 regarding natural phenomena are more certain than any- 

 thing else within the objective field. Nevertheless, the truth 

 in science must always be held open to extension or modifica- 

 tion, if not to complete replacement. 



