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the deception of the senses consists in their falsely 

 suggesting ideas of permanence. 



The test of truth used by each of these schools seems 

 to have been that what they pleased to assert was true, 

 and that all other opinion was mere ignorance. A 

 similar test has often been used in later times when 

 any particular school of thought has acquired a pre- 

 dominance, but the coexistence of the two rival 

 schools in the fifth century B.C. called attention to 

 the desirability of some less arbitrary criterion. 

 Accordingly Protagoras (480-410 B.C.) returned to 

 sensation as the test of truth, asserting dogmatically 

 that there is a fixed relation between the changes of 

 the outer world and the changes of sensation, and 

 that in this way " Man is the measure of truth." He 

 pushed this principle so far as to declare that what is 

 perceived by man exists ; what is perceived by no 

 man does not exist. Socrates, again (470-399 B.C.), 

 seems to have been of opinion that truth, which was 

 uncertain, or confused, in the complexity of concrete 

 instances, could always be clearly and certainly per- 

 ceived when a proposition was put into the form of a 

 generalisation. He applied his philosophy, however, 

 only to ethics, paying little attention to other 

 subjects. 



Both Plato and Aristotle show the influence of 

 the two last-named teachers, but in different de- 

 grees. 



Plato (427-347 B.C.) sometimes shows a leaning 



