A necessary condition for the just city is the “noble lie”. Given that the city is the soul writ large, a necessary condition for the just soul is the noble lie. The one discussed explicitly in the Republic is the “myth of the metals”. There is, however, another, one that Plato cleverly presented as a mystical realization, a transcendence known only to a few - the ascent out of the cave of ignorance in the light of the Good from which one sees the truth of the Forms and the Good itself.

When Socrates tells Glaucon about the turning of the soul to what is and the image of the Good, in response to Glaucon’s exclamation: “Apollo, what a demonic excess." Socrates’ response is:

"You … are responsible for compelling me to tell my opinions about it." (509c)

Opinions! Not the truth itself as he knows it to be, or even an image of the truth, but opinions.

At 532d Glaucon says:

So tell what the character of the power of dialectic is, and, then, into exactly what forms it is e divided; and finally what are its ways. For these, as it seems, would lead at last toward that place which is for the one who reaches it a haven from the road, as it were, and an end of his journey."
To which Socrates responds:

"You will no longer be able to follow, my dear Glaucon," I said, "although there wouldn't be any lack of eagerness on my part. But you would no longer be seeing an image of what we are saying, but rather the truth itself, at least as it looks to me. Whether it is really so or not can no longer be properly insisted on. But that there is some such thing to see must be insisted on. Isn't it so?"

Why can’t Socrates insist that the truth as it looks to him is the truth? And why should seeing "some such thing" be insisted on? The answer is because he does not have knowledge of the Forms. He has not escaped the cave. What has been disguised as revelations of esoteric, mystical truths, is a noble lie, a salutary public teaching. He has banished the poets and replaced them with his own philosophical poetry, new images on the cave wall.

Opinions! Not the truth itself as he knows it to be, or even an image of the truth, but opinions. — Fooloso4

Why can’t Socrates insist that the truth as it looks to him is the truth? And why should seeing "some such thing" be insisted on? The answer is because he does not have knowledge of the Forms. He has not escaped the cave. — Fooloso4

The character of Socrates as depicted by Plato is not always consistent with the historical personage of Socrates. Plato offered the largest body of material that included Socrates as a central figure. But when we cross reference the other accounts, we find that Plato took great liberty to embellish Socrate's philosophical scope.

I can only interpret Socrates theough the light of Socratic ignorance.

He knew nothing. He was not a skeptic, he was absolutely ignorant. So he made it his mission to find men who did know something. And through his method, he discovered these men did not know what they believed themselves to know. And even worse, he discovered that these renowned wisemen did not even know themselves.

Socrates came to reject the notion that "man is the measure of all things". Everything that can be said by a man is mere opinion. But he never assented to a knowledge of the forms, that was a Platonic fabrication. In fact, "The Republic" is entirely Platonic, not Socratic. Yet, if we were to examine Socrates in the terms of the cave, we could say he escaped his shackles, but remained in the cave to converse with the puppeteers about their shadow figures (Plato would be included amongst them). He did not leave the cave until he drank the hemlock.

The character of Socrates as depicted by Plato is not always consistent with the historical personage of Socrates. — Merkwurdichliebe

Right, I am referring here to Plato's depiction of Socrates, "made young and beautiful" (Second Letter). The Socrates depicted in the other two main sources, Xenophon and Aristophanes, is not historical either. But we should keep in mind Aristotle's claim that poetry is more philosophical than history.

He knew nothing. He was not a skeptic, he was absolutely ignorant. — Merkwurdichliebe

I agree he was a skeptic, although not in the modern sense of skepticism. True to his reputation for irony, he claims to know that he does not know, which is not the same as absolute ignorance.

And through his method, he discovered these men did not know what they believed themselves to know. — Merkwurdichliebe

Someone who knows nothing would not possess the skill needed to reveal the ignorance of others or to persuade the likes of Plato and Xenophon to learn from him. Plato's Socrates acknowledges the craftsmen's knowledge of their crafts. He also refers to the physician's knowledge, the ship captain's knowledge, and others who possess some form of knowledge. The problem is, being knowledgeable about one thing they wrongly believe they are knowledgeable about all others. Xenophon's Socrates also recognizes those who possess some form of knowledge.

But he never assented to a knowledge of the forms, that was a Platonic fabrication. — Merkwurdichliebe

The point of the passages I cited from the Republic is that he does not assent to knowledge of the Forms. It is not a Platonic fabrication, but an assumption the less than careful reader is led to. It should also be pointed out that in the Theaetetus, the dialogue devoted to the question of knowledge, there is no mention of the Forms.

In fact, "The Republic" is entirely Platonic, not Socratic. — Merkwurdichliebe

At one time, a great deal of effort was exerted attempting to distinguish Plato from Socrates. There are probably a few around who still try to identify the historical Socrates, but we simply do not have the evidence to do so. The problem is compounded by the fact that Plato never speaks in his own name in the dialogues. In the Seventh Letter he states:

But thus much I can certainly declare concerning all these writers, or prospective writers, who claim to know the subjects which I seriously study, whether as hearers of mine or of other teachers, or from their own discoveries; it is impossible, in my judgement at least, that these men should understand anything about this subject. There does not exist, nor will there ever exist, any treatise of mine dealing therewith. For it does not at all admit of verbal expression like other studies (341c)

Socrates came to reject the notion that "man is the measure of all things". — Merkwurdichliebe

We do not know this. If we look at Plato's dialogues, however, and after all the discussion is about Plato, things are not as simple as they may appear to be on the surface. If we do not have knowledge of the truth and must rely on opinion, then man is the measure. This does not mean that whatever man says is true but rather that in the absence of the truth as the measure we are left with what Socrates calls in Plato's Phaedo his "second sailing", that is, his reliance on speech. This is skeptical in a double sense. The Greek term 'skepsis' means both to inquire and to doubt. His reliance on speech is not an acceptance of whatever is said, but an inquiry or examination of it. It is sometimes referred to as zetetic skepticism. It is the method of dialectic.

Regarding the ‘noble lie’ - I am reminded of the Buddha’s ‘Parable of the Raft’. This parable compares the Buddha’s teaching to a raft, pulled together out of twigs and grass, used to ‘cross the river of suffering’, but to be discarded once the 'further shore' is reached. The parable concludes ‘Understanding the Dharma as taught compared to a raft, you should let go even of dharmas, to say nothing of non-dharmas’. So according to this parable, there’s a sense in which the teaching of the Buddha is only an expedient means, ‘thrown together’ out of what is at hand, and therefore on no account to be 'carried around'. I’m not sure that translates directly to a ‘noble lie’ but I think there is a kind of convergence. And it is something that is often reinforced in Mahayana Buddhism with the notion that the Buddha's speech is misleading, and that his real teaching was silence. This too issues in a dialectic, namely, the Madhyamika dialectic of Nāgārjuna.