Plato’s 2nd Letter, written after his 2nd journey to Sicily, is addressed to Dionysius II, tyrant of Syracuse. With reference to discussions on philosophy that Plato and Dionysius had held during Plato’s previous visit Plato writes: ‘You say that you have not had a sufficient demonstration of the doctrine concerning the nature of “the First” (ouch hikanȏs dedeichthai soi peri tês tou prȏtou physeȏs)… The matter stands thus (hȏde gar echei): Related to the King of All (peri ton pantȏn basilea) are all things (pant’ esti), and for his sake they are (kai ekeinou heneka panta), and of all things fair he is the cause (kai ekeino aition hapantȏn tȏn kalȏn).’ (312 D-E, tr. R. G. Bury). Bury says in his Prefatory Note to the Letter: ‘What is here said of “the King of All” is closely parallel to the description of the Idea of Good in Republic 509 B, D, 517 C; so it is natural to equate the First Principle and the first grade of Being with the Idea of Good.’ (Plato IX, LCL 234, pp. 400-401). Although I cannot but agree with Bury’s identification of “the King of All” with the Idea of Good, I have some quibbles. Plato’s text makes this identification a virtual certainty, for he slips from talking about the king, who is masculine (peri ton pantȏn basilea), to thinking of the Good, which is neuter (kai ekeino aition hapantȏn tȏn kalȏn). What Plato has here in mind is the Good of 509 B, D of the Republic, not the Idea of the Good of 517 C, which is feminine (hê tou akathou idea).
There is a profound difference between the passage concerning the Good in the 2nd Letter and the related passages in the Republic. The thought ‘all things are for the sake of the Good’ (kai ekeinou heneka panta) is missing in the Republic. The significance of this difference can be properly appreciated if we view it in the light of Aristotle’s criticism of Plato in Metaphysics A. at 988 a 9-11 Aristotle says that Plato ‘has used only two causes, that of the essence and the material cause (for the Forms are the causes of the essence of all other things, and the One is the cause of the essence of the Forms)’ (tr. W. D. Ross). Ross remarks that Aristotle ignores ‘various suggestions of a final cause – the ultimate good or hou charin of Philebus 20 D, 53 E, the object of the creator’s purpose in Timaeus 29 D ff., and in Laws 903 C.’ He says that Aristotle undoubtedly thought Plato’s treatment of this cause inadequate, but that that does not justify him in speaking as if Plato had ignored it entirely. (Aristotle’s Metaphysics, A revised text with Introduction and Commentary by W. D. Ross, Oxford University Press 1924, pp. 176-7).
In my entry of October 16, 2014 entitled ‘A note on the 3rd book of Aristotle’s Metaphysics’ I wrote: ‘The unceremonious manner with which Aristotle in the 1st book of the Metaphysics rejected the Forms on the basis of arguments marked in the Parmenides as irrelevant, while speaking about himself as one of Plato’s disciples – using the first person plural in the sense of “we Platonists” – indicates that he wrote the 1st book after Plato left Athens for Sicily and before he returned. Nobody expected that Plato would come back; he was in his late sixties when he went to Sicily, and he went there to help establish a state in which philosophers would rule.’
On this dating of Metaphysics A Ross’ criticism of Aristotle appears to be unjustified, for the Philebus, Timaeus, and Laws are late dialogues, which can be safely dated after Plato returned from his 3rd journey to Sicily. Plato’s attempts to do justice to the final cause in these three dialogues can be viewed as his response to Aristotle’s criticism.
What is then the significance of Plato’s presentation of the Good as the final cause in the 2nd Letter? I devoted my entry of November 14, 2014 to Siebeck’s conjecture that Plato in the Parmenides responded to Aristotle’s oral criticism of Plato’s theory of Forms in the Academy. If Plato’s 2nd Letter is authentic, it represents a powerful corroboration of Siebeck’s theory.