Plato dramatically staged the Parmenides so as to direct the minds of his disciples to the Republic and thus arm them against any criticism of the Forms (see the entry of January 10 2015 ‘Plato’s defence of the Forms in the Parmenides’). Aristotle’s passionate plea against the theory of Forms in the 9th chapter of the 1st book of Metaphysics indicates that Plato’s defence of the Forms in the Parmenides had a profound effect on his disciples: ‘Although philosophy on the whole seeks the cause of perceptible things (holȏs de zêtousês tês sophias peri tȏn phanerȏn to aition), we have given this up (for we say nothing about the cause from which change takes its start); we think that we are stating the substance of perceptible things when in fact we state the existence of other substances, while our account of the way in which they are the substances of perceptible things (hopȏs d’ ekeinai toutȏn ousiai) is empty talk (dia kenês legomen); for ‘sharing’ means nothing (to gar metechein outhen estin), as we said before’ (992a24-29).
In his response to the Parmenides, Aristotle too refers to the Republic: ‘Nor have the Forms any connection with what we claim to be the cause in the case of the sciences (hoper tais epistêmais horȏmen on aition), that for whose sake both all mind and the whole of nature are operative, with this cause which we assert to be one of the first principles; but mathematics has come to be identical with philosophy for the thinkers of today, though they say that it should be studied for the sake of other things (992a29-b1).’ Ross in his commentary rightly notes that Aristotle here refers to the Republic, where Plato views mathematics as a propaedeutic to philosophy (cf. Rep 533B-E; see Ross’ note ad Met. 992a33).
Concerning the words ‘what we claim to be the cause in the case of the sciences’ (hoper tais epistêmais horȏmen on aition) Ross says that ‘Difficulty has been felt about this, since science is concerned even more essentially with the formal than the final cause (note on 992a29)’. This difficulty disappears if we realize that in support of his own position Aristotle refers to Plato’s Republic. In the 6th book of the Republic Plato wrote that ‘the good is the cause of knowledge to all things known’ (tois gignȏskomenois to gignȏskesthai hupo tou agathou pareinai, 509b6-8).’
Aristotle’s criticism of the theory of Forms in the 1st book of Metaphysics is uncompromising: ‘Those who posit the Ideas (hoi de tas ideas tithemenoi) as causes, firstly, in seeking to grasp the causes of things around us, they introduced others equal in number to these, as if a man who wanted to count things thought he would not be able to do it while they were few, but tried to count them when he had added to their number (990a34-b4) … Further, of the ways in which we prove that the Forms exist, none is convincing; for from some no inference necessarily follows, and from some arise Forms even of things of which we think there are no Forms (990b9-11) … Above all one might discuss the question what on earth the Forms contribute to sensible things, either to those that are eternal [i. e. the heavenly bodies – W. D. Ross’ note ad 991a9] or to those that come into being and cease to be. For they cause neither movement nor any change in them. But again they help in no wise either towards the knowledge of the other things (for they are not even the substance of these, else they would have been in them), or towards their being, if they are not in the particulars which share in them’ (991a8-14, tr. Ross).
But Plato in the Parmenides gave voice to, dramatically displayed and enhanced the difficulties in which the theory of Forms is entangled, in particularly the difficulties involved in the theory of participation of perceptible things in the Forms and difficulties concerning the contribution the Forms can possibly make to our knowledge of things around us (133b-134e). Plato not only conceived the Forms face to face with all this criticism of the theory, criticism of which he was well aware from the time he became interested in philosophy, but steadfastly adhered to them – this is what the Parmenides is all about. How was this possible? The answer lies in the way in which Plato conceived the Forms, as Aristotle explains it in Metaphysics A.
Aristotle says that Plato in his youth embraced the Heraclitean doctrines ‘that all things are in constant flux (hȏs hapantȏn aei reontȏn) and there is no knowledge about them’ (kai epistêmês peri autȏn ouk ousês, 987a33-34). To understand the significance of this statement, we must pay due attention to the Greek concept of knowledge. Epistêmê signifies ‘standing on’; it is derived from ephistêmi, ‘stop, cause to halt’. Aristotle says in the Physics: ‘for it is when the mind has reached a state of rest and come to a standstill (tȏi gar êremêsai kai stênai tên dianoian) that we say we know and understand (epistasthai kai phronein legometha, 247b11-12)’. Engrossed in the Heraclitean view of reality, Plato encountered Socrates ‘who was the first to have stopped and fixed his mind on definitions of ethical concepts’ (peri horismȏn epistêsantos prȏtou tên dianoian, Met A, 987b3-4); Plato realized ‘that the entities on which mind could be thus fixed and brought to a standstill were different from perceptible things (hȏs peri heterȏn touto gignomenon kai ou tȏn aisthêtȏn). He called the entities of this kind Forms’ (houtos men oun ta men toiauta tȏn ontȏn ideas prosêgoreuse, 987b5-8).
Plato did not conceive the Forms on the basis of observations concerning things around him as young Socrates did in the Parmenides and as did Plato’s disciples at whom Aristotle directed his criticism. On his encounter with Socrates, Plato saw the Forms with his soul’s eye (cf. to tês psuchês omma, Rep. 533d2). After Plato had conceived the Forms, his Heraclitean view of the perceptible world remained unchanged: ‘this view he held even later’ (tauta men kai husteron houtȏs hupelaben, Met. A 987a34-b1), that is after he had conceived the Forms. In the Republic Plato insists that there can be no knowledge concerning things perceived by the senses (508d-511e); this is why his view of the Forms was immune to Aristotelian arguments against the Forms.
Face to face with the Parmenides Aristotle was well aware that his arguments against the Forms were powerless against Plato’s view of the Forms; he directed his arguments against the Forms at those, who like himself believed that ‘philosophy on the whole seeks the cause of perceptible things’ (holȏs de zêtousês tês sophias peri tȏn phanerȏn to aition, Met. A 992a24-25).