At least twice the Platonic corpus, there are sequences of three dialogues with visible connections to one another and which indicate the existence of a fourth dialogue that is either lost of left out.
1. The sequence Republic – Timaeus – Critias – (Hermocrates)
2. The sequence Theaetetus – Sophist – Statesman – (Philosophos)
(I have indicated the “missing” dialogues in parentheses.)
What shall we make of this? There are two obvious explanations for the the missing dialogues: either they were lost or they were never written. Either answer is possible, but the defective reader in me wants to favor the latter explanation as the more fruitful. Perhaps a missing dialogue asks us to look closer at the other three in order to find the missing fourth intended and indicated by the previous three. Perhaps we are asked to read these sequences defectively, to pay attention to the felt absence of a satisfying whole/end that is presupposed by their defectiveness.
Now, I can’t in a blog post give any more than hints, but here are a few clues toward reading at least the first of the existing trilogies defectively:
The sequence Republic – Timaeus – Critias – (Hermocrates)
a) The Republic gives no forward pointer to future engagements, but early in the Timaeus there is a recapitulation of a city-in-speech unmistakably that of the Republic.
b) The Timaeus begins with the words, “One, two, three,—but where, my dear Timaeus, is the fourth of our guests of yesterday, our hosts of today?” (Perseus Project translation, 17a) In an earlier discussion of the Divided Line image, I claimed that it is a “protreptic analogy” — three related terms in search of a missing fourth.
c) The Timaeus’ recapitulation of the city-in-speech is a version that takes no notice of the philosopher rulers or the Idea of the Good. It is a truncated version, a version that is based almost completely on what I have called the “Second Draft” version.
d) In the Timaeus, Socrates indicates a defect in the (truncated) city-in-speech:
“And now, in the next place, listen to what my feeling is with regard to the polity we have described. I may compare my feeling to something of this kind: suppose, for instance, that on seeing beautiful creatures, whether works of art or actually alive but in repose, a man should be moved with desire to behold them in motion and vigorously engaged in some such exercise as seemed suitable to their physique; well, that is the very feeling I have regarding the State we have described.” (Perseus Project translation, 19b-c)
d) The Critias appears to be fragmented. Again, I want to hypothesize that this is intentional. It ends with the character Critias describing the corruption of the civilization of Atlantis and Zeus about to announce his solution to the assembled gods:
Such was the magnitude and character of the power which existed in those regions at that time; and this power the God set in array and brought against these regions of ours on some such pretext as the following, according to the story. For many generations, [120e] so long as the inherited nature of the God remained strong in them, they were submissive to the laws and kindly disposed to their divine kindred. For the intents of their hearts were true and in all ways noble, and they showed gentleness joined with wisdom in dealing with the changes and chances of life and in their dealings one with another. Consequently they thought scorn of everything save virtue and lightly esteemed their rich possessions, bearing with ease [121a] the burden, as it were, of the vast volume of their gold and other goods; and thus their wealth did not make them drunk with pride so that they lost control of themselves and went to ruin; rather, in their soberness of mind they clearly saw that all these good things are increased by general amity combined with virtue, whereas the eager pursuit and worship of these goods not only causes the goods themselves to diminish but makes virtue also to perish with them. As a result, then, of such reasoning and of the continuance of their divine nature all their wealth had grown to such a greatness as we previously described. But when the portion of divinity within them was now becoming faint and weak through being ofttimes blended with a large measure of mortality, [121b] whereas the human temper was becoming dominant, then at length they lost their comeliness, through being unable to bear the burden of their possessions, and became ugly to look upon, in the eyes of him who has the gift of sight; for they had lost the fairest of their goods from the most precious of their parts; but in the eyes of those who have no gift of perceiving what is the truly happy life, it was then above all that they appeared to be superlatively fair and blessed, filled as they were with lawless ambition and power. And Zeus, the God of gods, who reigns by Law, inasmuch as he has the gift of perceiving such things, marked how this righteous race was in evil plight, and desired to inflict punishment upon them, to the end that when chastised they might strike a truer note. [121c] Wherefore he assembled together all the gods into that abode which they honor most, standing as it does at the center of all the Universe, and beholding all things that partake of generation and when he had assembled them, he spake thus: … [Text breaks off here.] (Perseus Project translation, 120d-121c)
e) So the dialogue ends with the silence of the God, but an anticipation of his decree. On the defective reading, the natural response is “Hmmm!”
f) The next scheduled speaker would be Hermocrates, hence his name is ascribed to the missing dialogue. The name means “Hermes’ power” — in other words, the power to deliver a god’s words to man. And Critias breaks off with Zeus about to speak. Is the direct speech of the god such that it can only be inscribed on the soul of the philosopher? Perhaps…
I personally do not favor the imposition of “writing periods” on Plato’s corpus: the suggestion, almost a dogma in some circles, that there was a early, middle and late period. The fact that Plato expands into defective trilogies dialogues such as Republic and Theaetetus suggests that they never were far from his mind. There is even an ancient report of Dionysius of Halicarnassus that says,
“Plato kept on combing and curling and in every way braiding his dialogues even when he had turned eighty.”
I think it is more likely that the dialogues differ in character because the characters themselves differ. I am however sympathetic with William Altman that there was an intended “reading order” of the dialogues, an order motivated by pedagogical effectiveness. See his book Plato the Teacher: The Crisis of the Republic.