Why should the denial of it strike Protagoras as 'incredible' ( 170 c 9) ?sx The answer is clear from what was said above in explanation of the slogan 'Man is the measure .sx ' The import of that slogan is not just that people believe all and only truths , but also that we may apply ordinary criteria for deciding who believes what .sx Furthermore Protagoras has all along implicitly acknowledged that when we do apply those ordinary criteria we find people believing the sorts of thing that Plato observes people to believe .sx For if it were not evident that ( to put it at its mildest ) people hold different and conflicting beliefs , there would have been no need for Protagoras to introduce qualifiers and speak of things being true for people ; plain unqualified truth would have done instead .sx There is thus no doubt that Protagoras will join Plato in making the observation with which Plato's argument begins , the observation that people think Protagoras wrong .sx There is a second contrast between Plato's own argument and the argument that a belief that there are false beliefs guarantees its own truth .sx When people think that people differ from one another in what they know , then they think , not only that some people make mistakes which others manage to avoid , but also that some people have true beliefs which not all others share .sx This , I take it , is the import of the remarks in 170 b 6-9 about knowledge , ignorance , truth and falsehood .sx The consequence is that if Plato manages to vindicate this generally held belief , he will have established that Protagoras is wrong on two counts , rather than just on one ; he will have established not only that one's beliefs can be untrue , but also that there can be truths which one does not believe .sx The third contrast is this .sx Plato observes not just that someone , but that everyone , save perhaps for Protagoras himself , thinks Protagoras wrong ; there is , Plato observes , an almost universal consensus of opinion against Protagoras .sx There was no parallel to this in our argument concerning the belief that at least one belief is not true .sx If that argument works at all , then it works even if only one person believes that there are untrue beliefs ; and it certainly does not work any better if that belief is shared by lots of people .sx Plato's argument does by contrast need the premiss that , with the possible exception of Protagoras himself , everyone thinks that man is not the measure .sx It needs this premiss for two reasons , one slight and one serious .sx The slight reason is that only if Protagoras is outnumbered can Plato infer , as he does in the aside at 171 a 1-3 , that Protagoras' doctrine is more untrue than true .sx The serious reason is that unless it is already accepted that no one else agrees with Protagoras' doctrine , getting Protagoras to disagree with it will not mean that the consensus against Protagoras' doctrine is completely unanimous .sx Yet complete unanimity is what Plato wants :sx he means to prove that Protagoras' doctrine is not true for anyone whatsoever ( 171 c 6-7 ; compare 170 e 9 - 171 a 1) .sx Only thus can Plato drop all qualifications and call Protagoras' doctrine just plain untrue , without any fear of the riposte :sx " Some people think it untrue ; but others think otherwise ; and might not both sides be right ?sx " .sx How can Protagoras be encouraged to join the consensus that man is not the measure ?sx Perhaps he does not need to ; perhaps he is already party to that consensus anyway .sx If so , we can proceed at once to the conclusion that Protagoras' doctrine is not true for anyone , and thus is not true at all ( 170 e 7 - 171 a 1) .sx If not , there is a little more work to do .sx Plato undertakes that extra work at 171 a 6 - c 3 .sx Protagoras must , says Plato , agree that those who disagree with him think the truth when they think him mistaken ; and so he must himself agree he is mistaken .sx The converse , Plato points out , does not hold :sx people other than Protagoras are under no corresponding pressure to agree with him and think themselves wrong ( 171 b 4-5) .sx Why does Plato bother to point this out ?sx Perhaps Protagoras might otherwise try to declare a draw on the grounds that his opponents are in a position exactly like his own ; and might then try to urge that this draw is yet another illustration of how man is the measure .sx If it was intended to stop Protagoras defending himself in this way , then Plato's remark is a success .sx Those who disagree with Protagoras can hardly be expected to accept also that his doctrine is true , or true for them , or even true for Protagoras himself ; after all , qualified truth , truth-for , is Protagoras' notion , not theirs .sx Thus while the opposition need not in any way accept the doctrine of Protagoras , Protagoras must agree with the opposition that his doctrine is false .sx And thus the doctrine of Protagoras is not true for anyone , not even for Protagoras himself .sx Few readers of the Theaetetus feel entirely satisfied by this .sx Protagoras is made to concede that the views of his opponents are true .sx That , readers rightly feel , is a big concession .sx Need Protagoras make it ?sx Can he not simply say that what others think on this topic is true for them , and for them alone ?sx Why need Protagoras himself accept as true what other people think on this topic ?sx Is Plato being fair when he has Protagoras accept the other side's belief as true , instead of merely true for the other side ( 171 a 8 , b 2 , b 6 , b 11 ; compare b 1) ?sx Yes , Plato is being fair .sx Protagoras cannot here concede only that the opinion of those who disagree with him is true for them .sx He must go further , and speaking for himself say that their opinion is true .sx This can be shown even if for the moment we allow that some beliefs can be true for one person without being true for every other person ; for even if some beliefs can do that , the belief that man is not the measure could not be among them .sx Consider an analogy ; 'Traffic keeps left' is , if you care to put it that way , true in Wales , and not true in Canada ; but such things as 'Traffic keeps left everywhere' , 'Traffic keeps left somewhere' , or 'Traffic keeps left in Canada' , purport to be true no matter what the country in which they are said ; such a thing therefore could not be true in only some countries ; such a thing would have to be either true everywhere or else true nowhere whatsoever .sx As with truth in places , so too with truth for people .sx If a belief purports to be true for every person , it cannot be true for only some ; such a belief would have to be either true for everyone , or else true for no one whatsoever .sx This applies to the widespread consensus that Protagoras is wrong .sx Those party to the consensus hold , among other things , that if the best that can be said for a belief is that it is their belief , then that belief is not true .sx Now you cannot think such a thing , and think also that the best that can be said for it is that you think it .sx It would be absurd for me to say 'If something is only my opinion , then it is not true ; but mind you , this is only my opinion .sx ' The belief that Protagoras is wrong therefore purports to be something more than just true for those who happen to accept it .sx That is why Protagoras cannot simply say that the opinion of his opponents is true for them , and stop there ; that is why he must go on to say for himself that their opinion is true .sx But once Protagoras does that , there is a universal consensus that it is , without qualification , false to say that man is the measure .sx And so we can conclude that man is not the measure , full stop .sx Why not everything flows .sx Protagoras' theory of relative truth may therefore be rejected .sx When however the Secret Doctrine was first expounded , Protagoras' theory of truth was connected with a metaphysic summarised in the slogan 'all things flow' .sx That metaphysic implies that all our thoughts , even those that apparently contradict one another , are nevertheless equally true .sx That metaphysic therefore cannot be allowed to stand .sx For if we allow that metaphysic to stand , then our refutation of Protagoras will mean only that the truth which equally characterises all our thoughts is not a relative truth , but an absolute one , unqualified by any reference to their thinkers .sx And if any thought is guaranteed to be unqualifiedly true just because it is thought , then that threatens objectivity no less than a guarantee that any thought is true for its thinker .sx Plato has a fairly brisk way with the idea that 'all things flow' .sx Those who accept that slogan should accept , says Socrates , that everything is changing in every way at every time ( Theaetetus 182 a 1) .sx The 'every' here is to be taken with the utmost strictness .sx It is not just that we have , for example , Socrates change from seeing to not seeing the whitewashed wall as white , and the whitewashed wall change from being white for Socrates to not being white for him .sx It is not only objects , whether perceiving or perceived , that change ; perceptions and perceived qualities must change too .sx Thus Socrates' glimpse of the wall too must be changing , from being a glimpse to not being a glimpse ; and likewise the whiteness which makes the wall white for Socrates cannot be stably a whiteness , but must itself be changing ( 182 d 1 - e 5) .sx We will later explore the consequences of this completely universal change .sx In the meantime , let us ask why Plato insists that those who accept the slogan 'all things flow' must believe the particularly extreme version of that doctrine which he expounds .sx Plato does in fact offer us some argument for his insistence .sx At 181 e 5-8 he points out that if any less extreme version were true , then saying 'everything moves' would be no more correct than saying 'everything is still' .sx The trouble is that this argument sounds less than conclusive .sx It invites the reply :sx 'all things flow' is only a slogan ; the doctrine which we mean by that slogan is the one summarised so well at Theaetetus 156 a 2 - 157 c 2 ; according to this doctrine of ours , the 'things' which 'flow' are perceiving subjects , such as Socrates , and perceived objects , such as walls ; the sense in which a perceiving subject 'flows' is that Socrates , who perceives the wall as white now , can nevertheless perceive other things differently , and can even perceive the wall differently at some other time ; likewise , the sense in which a perceived object 'flows' is that the wall , which now is white for Socrates , can nevertheless have other colours for other people , and can even have other colours for Socrates himself at some other time ; perceived qualities and perceptions of those qualities are not themselves among the 'things' which , according to our doctrine , 'flow' ; so perhaps 'all things flow' is therefore a poor slogan for our own modest doctrine , and a better slogan for a doctrine far more extreme ; but at most that requires us to abandon the slogan ; it hardly requires us to accept the extreme doctrine ; and in that case , Plato's refutation of the doctrine that 'all things flow' is a refutation of a doctrine that nobody either does or need hold ; Plato has just invented an easy target for himself .sx Such at any rate is the accusation which Plato's arguments invite .sx But is Plato guilty as charged ?sx Plato is innocent ; there are sound reasons for him to formulate and refute the extreme doctrine that 'everything is changing in every way at every time' .sx The first reason is that only the extreme doctrine coheres with Protagoras' claim that man is the measure of all things .sx