When Socrates proved unable to defend the Forms against the difficulties raised by Parmenides, the latter pointed out to him that ‘there are many other difficulties (polla men kai alla), but the greatest is this (megiston de tode): If someone should say that the Forms cannot be known (Ei tis phaiê mêde prosêkein auta gignȏskesthai, 133b4-5) … ‘those Forms that are what they are in their relation to one another (hosai tȏn ideȏn pros allêlas eisin hai eisin) have their being in their relation to themselves (autai pros hautas tên ousian echousin), but not in relation to things among us (all ou pros ta par hêmin), resemblances, or whatever one may posit them to be (eite homoiȏmata eite hopêi dê tis auta tithetai), which we possess and call to be (hȏn hêmeis metechontes eiani hekasta eponomazometha); and the things that are among us and have the same name with the Forms (ta de par hêmin tauta homȏnuma onta ekeinois) are again related only to one another (auta au pros hauta estin), but not to the Forms (all’ ou pros ta eidê), and belong to themselves (kai heautȏn) but not to the Forms that have the same name again (all’ ouk ekeinȏn hosa au onomazetai houtȏs, 133c8-d5).’
Plato had good reasons to call this the greatest difficulty concerning the Forms. Aristotle writes in Metaphysics A: ‘Above all one might discuss the question (pantȏn de malista diaporêseie an tis) what on earth the Forms contribute (ti pote sumballetai ta eidê) to sensible things, either to those that are eternal or to those that come into being and cease to be (tois aidiois tȏn aisthêtȏn ê tois gignomenois kai phtheiromenois). For they cause neither movement nor any change in them (oute gar kinêseȏs oute metabolês oudemias estin aitia autois). But again they help in no wise either towards the knowledge of the other things (alla mên oute pros tên epistêmên outhen boêthei tên tȏn allȏn), for they are not even the substance of these (oude gar ousia ekeina toutȏn), else they would have been in them (en toutois gar an ên), or towards their being (oute eis to einai), if they are not in the particulars which share in them (mê enuparchonta tois metechousin).’ (991a8-14, tr. W. D. Ross; Ross notes on tois aidiois tȏn aisthêtȏn: ‘the heavenly bodies’.)
Plato did not shrink from voicing the grave consequences of this objection. Parmenides: ‘So none of the Forms is known to us (Ouk ara hupo ge hêmȏn gignȏsketai tȏn eidȏn ouden), since we do not have a share of knowledge itself (epeidê autês epistêmês ou metechomen).’ – Socrates: ‘It seems not (Ouk eoiken).’ – Parmenides: ‘Unknowable to us is therefore beautiful itself (Agnȏston ara hêmin kai auto to kalon), what it is (ho esti), and the good (kai to agathon), and everything we assume as being the Forms themselves (kai panta ha dê hȏs ideas autas ousas hupolambanomen).’ (134b11-c2)
That Plato did not see this objection as relevant is clear from the way in which he introduces and closes the discussion of it in the Parmenides. But it is equally clear that Aristotle persisted in considering it an insurmountable difficulty for the theory especially concerning its ethical aspects.
In the Eudemian Ethics Aristotle says: ‘Let us say that in the first place to assert the existence of a Form not only of good (legȏmen hoti prȏton men to einai idean mê monon agathou) but of anything else (alla kai allou hotououn) is empty talk (legetai logikȏs kai kenȏs) … next (epeita), even granting that Forms and the Form of good do exist (ei kai hoti malist’ eisin hai ideai kai agathou idea), it can’t be of any practical value for the good life or for actions (mê pot’ oude chrêsimos pros zȏên agathên oude pros tas praxeis). (1217b20-25)
In the Nicomachean Ethics Aristotle introduces the problem as follows: ‘We had perhaps better consider the universal good and discuss thoroughly what is meant by it (To de katholou beltion isȏs episkepsasthai kai diaporêsai pȏs legetai), although such an inquiry is made an uphill task (kaiper prosantous tês toiautês zêtêseȏs gignomenês) by the fact that the Forms have been introduced by friends of our own (dia to philous andras eisagagein ta eidê). Yet it would perhaps be thought to be better (doxeie d’ an isȏs beltion einai), indeed to be our duty (kai dein), for the sake of maintaining the truth (epi sȏtêriai ge tês alêtheias) even to destroy what touches us closely (kai ta oikeia anairein), especially as we are philosophers or lovers of wisdom (allȏs te kai philosophous ontas); for, while both are dear (amphoin gar ontoin philoin), piety requires us to honour truth above our friends (hosion protiman tên alêtheian).’ (1096a11-17, tr. W. D. Ross)