Chap. II. ANTIENT METAPHYSICS. 327 



fay, that fuch a confufed complicated notion (and fuch all the percep- 

 tions of fenfe are) is a proper fubjedt of a fcience of fuch certainty as 

 geometry ? 



But, further, upon the fuppofnion that we have only perceptions of 

 fenfe, weaker or ftronger, there not only can be no fcience, but no 

 truth or falfehood ; becaufe there can be no criterion of either j or, if 

 there be any, every man muft be the ftandard of truth to himfelf, ac- 

 cording to the dodrine of the antient fophift Protagoras ; and every 

 thing will be true which appears to be fo to every man. Now, as things 

 appear very differently to different men, and to the fame men at dif- 

 ferent times, the confequence neceffarily is, that the fame thing will be 

 both true and falfe ; or, in other words, that there is no fuch thing as 

 either truth or falfehood. The foundation of this opinion, as Ari- 

 ftotle informs us *, is no other than the dodrine maintained by thofe 

 among us who deny the exiftence of ideas, viz. that we have no 

 knowledge except fenfation, nor, confequently, any other criterion of 

 truth or falfehood befides our fenfes. Now, as fenfation depends not 

 only upon the operations of external objeds upon our organs of fenfe, 

 but upon the difpofuion of thefe organs, it will often happen that, 

 though the operation of external objeds be the fame, yet the fenfa- 

 tion will be very different, by means of the different difpofition of the 

 organs, not only in different men, but in the fame man at different 

 times. And, befides the difpofuion of the organs, there are, in thofe 

 fenfes which perceive things at a diftance, fuch as fight and hearing, 

 different difpofitions of the media through which fuch fenfes operate ; 

 and thefe, as is well known, make a great difference in their percep- 

 tions. So that, according to this philofophy, truth or fcience is a mere 

 fpedre or phantom, appearing differently to different men, or to the 

 fame man at different times, without any fubffance or reality. But, 

 the true anfwer to Heraclitus, Protagoras, or whoever elfe, antient or 



modern^ 



♦ Metaph. lib. 4. cap. ^^ 



