[Footnote 29: Plato, Parmenid. 133 B. [Greek: ei)/ tis phai/ê mêde\ prosê/kein au)ta\ gignô/skesthai o)/nta toiau=ta oi(=a/ phamen dei=n ei)=nai ta\ ei)/dê. . . . a)pi/thanos a)\n ei)/ê o( a)/gnôsta au)ta\ a)nagka/zôn ei)=nai.] 134 A. [Greek: ê( de\ par' ê(mi=n e)pistê/mê ou) tê=s par' ê(mi=n a)\n a)lêthei/as ei)/ê? kai\ au)= e(ka/stê ê( par' ê(mi=n e(pistê/mê tô=n par' ê(mi=n o)/ntôn e(ka/stou a)\n e)pistê/mê xu/mbainoi ei)=nai?] 134 C. [Greek: a)/gnôston a)/ra ê(mi=n e)/sti kai\ au)to\ to\ kalo\n o(\ e)/sti, kai\ to\ a)gatho/n, kai\ pa/nta a(\ dê\ ô(s i)de/as au)ta\s ou)/sas u(polamba/nomen.]] The opinion here advanced by the Platonic Parmenides asserts, in other words, what is equivalent to the memorable dictum of Protagoras--"Man is the measure of all things--of things existent, that they do exist--and of things non-existent, that they do not exist". This dictum affirms universal relativity, and nothing else: though Plato, as we shall see in the elaborate argument against it delivered by Sokrates in the Theætêtus, mixed it up with another doctrine altogether distinct and independent--the doctrine that knowledge is sensible perception.[30] Parmenides here argues that if these Forms or Ideas are known by us, they can be known only as relative to us: and that if they be not relative to us, they cannot be known by us at all. Such relativity belongs as much to the world of Conception, as to the world of Perception. And it is remarkable that Plato admits this essential relativity not merely here, but also in the Sophistês: in which latter dialogue he denies the Forms or Ideas to be absolute existences, on the special ground that they are known:--and on the farther ground that what is known must act upon the knowing mind, and must be acted upon thereby, _i.e._, must be relative. He there defines the existent to be, that which has power to act upon something else, or to be acted upon by something else. Such relativeness he declares to constitute _existence_:[31] defining existence to mean potentiality. [Footnote 30: I shall discuss this in the coming chapter upon the Theætêtus.] [Footnote 31: Plato, Sophistês, pp. 248-249. This reasoning is put into the mouth of the Eleatic Stranger, the principal person in that dialogue.] [Side-note: Answer of Sokrates--That Ideas are mere conceptions of the mind. Objection of Parmenides correct, though undeveloped.]