In the Parmenides Plato maintains that only a man of considerable natural gifts (135a7), if he is willing to undergo a course of prolonged study, beginning ‘from afar’ (porrôthen, 133b9), can be shown that the arguments against the Forms are false (133b7). With these words he points to the Republic taken as a whole; the actual road to the Forms starts in the latter part of the fifth book and culminates at the sight of the Form of Good in the sixth book.
In the Parmenides we meet with Adeimantus and Glaucon in the opening line of the dialogue. While these two brothers of Plato link the Parmenides to the Republic, where they figure as Socrates’ main interlocutors, Glaucon on his own joins the Parmenides to the Symposium, in the opening paragraph of which he figures as a man eager to listen to speeches on love (peri tôn erôtikôn logôn tines êsan, 172b2-3) and as a man who thinks he ought to do anything rather than engage in philosophy (panta mallon prattein ê philosophein, 173a3). This characterization of him points us to the fifth book of the Republic, where he gives vent to his deprecatory view of philosophy (173e6-174a4) and where Socrates characterizes him as a man interested in everything concerning love (anêr erôtikos, 474d4). In this way Plato directs the eye of the reader of the Parmenides to the point in the fifth book of the Republic at which the road to the Forms begins, and where we can learn, why Plato can view as solved the problems raised by the arguments against the Forms raised in the former by pointing to the latter.
In Republic V Socrates contrasts a philosopher who recognizes the existence of beauty itself (hêgoumenos te ti auto kalon), i.e. of the Form of Beauty, and is able to see it (kai dunamenos kathoran kai auto) and the objects which participate in it (kai ta ekeinou metechonta, 476c9-d2), with a man who loves beautiful things (kala pragmata nomizȏn) but has no sense of beauty itself (auto de kallos mê nomizȏn, 476c2-3). The state of mind of the former is properly called knowledge (toutou men tên dianoian hȏs gignȏskontos gnȏmên an orthȏs phaimen einai), that of the latter, who opines only, opinion (tou de doxan hȏs doxazontos, 476d5-6). Being that fully is (to men pantelȏs on) is fully knowable (pantelȏs gnȏston), not-being (mê on de mêdamê) is utterly unknown (pantêi agnȏston, 477a2-3); opinion (doxa) is in between these two (metaxu toutoin, 478d3). Doxa is related to that which partakes equally of being and not-being (to amphoterȏn metechon, tou einai te kai mê einai), and cannot rightly be termed either (kai oudeteron eilikrines orthȏs an prosagoreuomenon, 478e1-3).
Socrates: ‘This being premised (toutȏn de hupokeimenȏn), I would ask the gentleman who is of opinion that there is no absolute or unchangeable Idea of beauty, but only a number of beautiful things – he, I say, your lover of beautiful sights, who cannot bear to be told that the beautiful is one, and the just is one, or that anything else is one – to him I would appeal, saying (legetȏ moi, phêsȏ, kai apokrinesthȏ ho chrêstos hos auto men to kalon kai idean tina autou kallous mêdemian hêgeitai aei men kata t’auta hȏsautȏs echousan, polla de ta kala nomizei, ekeinos ho philotheamȏn kai oudamêi anechomenos an tis hen to kalon phêi einai kai dikaion kai t’alla houtȏ), Will you be so very kind, sir, as to tell us whether, of all these beautiful things, there is one which will not be found ugly (Toutȏn gar dê, ȏ ariste, phêsomen, tȏn pollȏn kalȏn mȏn ti estin ho ouk aischron phanêsetai); or of the just, which will not be found unjust (kai tȏn dikaiȏn ho ouk adikon); or of the holy, which will not also seem unholy (kai tȏn hosiȏn, ho ouk anosion)?’ – Glaucon: ‘No (Ouk), these things must (all’ anankê), from different points of view, be found both beautiful and ugly (kai kala pȏs auta kai aischra phanênai); and the same is true of the rest (kai hosa alla erȏtas).’ (478e7-479b2) – Socrates: ‘Thus then we seem to have discovered (Hêurêkamen ara, hȏs eoiken) that the many notions which the multitude entertains (hoti ta tȏn pollȏn polla nomima) about the beautiful (kalou te peri) and about all other things (kai tȏn allȏn) are tossing about in some region which is half-way (metaxu pou kulindeitai) between pure being and pure not-being (tou te mê ontos kai tou ontos eilikrinȏs).’ – Glaucon: ‘We have (Hêurêkamen).’ (479d3-6) (Tr. B. Jowett)
In the light of this discussion between Socrates and Glaucon in the Republic, all objections raised against the Forms in the Parmenides are shown to be made by a man whose mind is wandering between knowledge and ignorance, deprived of the former, so that whatever he may say against the Forms is false (hoti pseudetai, Parm. 133b7), he only seems to be saying something, while in fact he says nothing (tauta legonta dokein te ti legein, Parm. 135a5-6), there is nothing sound in what he says (ouch hugiainei, Rep. 476e2).