presuppose，toguardagainstdialecticalobjections，anyfurther
qualificationswhichmightbeadded。This，then，isthemostcertain
ofallprinciples，sinceitanswerstothedefinitiongivenabove。For
itisimpossibleforanyonetobelievethesamethingtobeandnot
tobe，assomethinkHeraclitussays。Forwhatamansays，hedoesnot