It is the IDEA of good.

And the steps of the ladder leading up to this highest or universal existence are the mathematical sciences, which also contain in themselves an element of the universal.

These, too, we see in a new manner when we connect them with the idea of good.

They then cease to be hypotheses or pictures, and become essential parts of a higher truth which is at once their first principle and their final cause.
We cannot give any more precise meaning to this remarkable passage, but we may trace in it several rudiments or vestiges of thought which are common to us and to Plato: such as (1) the unity and correlation of the sciences, or rather of science, for in Plato's time they were not yet parted off or distinguished; (2) the existence of a Divine Power, or life or idea or cause or reason, not yet conceived or no longer conceived as in the Timaeus and elsewhere under the form of a person; (3) the recognition of the hypothetical and conditional character of the mathematical sciences, and in a measure of every science when isolated from the rest; (4) the conviction of a truth which is invisible, and of a law, though hardly a law of nature, which permeates the intellectual rather than the visible world.
The method of Socrates is hesitating and tentative, awaiting the fuller explanation of the idea of good, and of the nature of dialectic in the seventh book.

The imperfect intelligence of Glaucon, and the reluctance of Socrates to make a beginning, mark the difficulty of the subject.
The allusion to Theages' bridle, and to the internal oracle, or demonic sign, of Socrates, which here, as always in Plato, is only prohibitory; the remark that the salvation of any remnant of good in the present evil state of the world is due to God only; the reference to a future state of existence, which is unknown to Glaucon in the tenth book, and in which the discussions of Socrates and his disciples would be resumed; the surprise in the answers; the fanciful irony of Socrates, where he pretends that he can only describe the strange position of the philosopher in a figure of speech; the original observation that the Sophists, after all, are only the representatives and not the leaders of public opinion; the picture of the philosopher standing aside in the shower of sleet under a wall; the figure of 'the great beast' followed by the expression of good-will towards the common people who would not have rejected the philosopher if they had known him; the 'right noble thought' that the highest truths demand the greatest exactness; the hesitation of Socrates in returning once more to his well-worn theme of the idea of good; the ludicrous earnestness of Glaucon; the comparison of philosophy to a deserted maiden who marries beneath her--are some of the most interesting characteristics of the sixth book.
Yet a few more words may be added, on the old theme, which was so oft discussed in the Socratic circle, of which we, like Glaucon and Adeimantus, would fain, if possible, have a clearer notion.