“I know that our efforts all come to nothing. Analyze life, tear its trappings off, lay it bare with thought, with logic, with philosophy, and its emptiness is revealed as a bottomless pit; its nothingness frankly confesses to nothingness, and Despair comes to perch in the soulI know the end of us all is nothing, I know that at the end of Time, the reward of our toil will be nothing — and again nothing. I know that all our handiwork and all our ideas will be destroyed. I know that not even ash will be left from the fires that consume us. I know that our ideals, even those we achieve, will vanish in the eternal darkness of oblivion and final non-being. There is no hope, none, in my heart. I know, No promise, none, can I make to myself and to others. No recompense can I expect for my labors. No fruit will be born of my thoughts. I know the time — eternal seducer of all men, eternal cause of all effects — offers me nothing but the blank prospect of annihilation. So, my dignity is broken and weak, in recognition of my impending defeat.

The man who is alone, who stands on his own feet, who is stripped bare, who asks for nothing and wants nothing, who has reached the apex of disinterested­ness not through blind renunciation but through ex­cess of clear vision, turns to the world which stretches out before him as a burned prairie, as a devastated city — a world in which no churches, asylums, refuges, ideals, are left — and says: «Though you promise me nothing I am still with you, I am still an atom of your energies, my work is part of your work; I am your companion and your mirror as you march on your merciless way. But I owe nothing to any one. I would be responsible to freedom alone.”

Our Eternity, reported in Bartlett's Familiar Quotations, 10th ed. (1919)

See All I know is that I know nothing on Wikipedia for a detailed account of the origins of this attribution.

μοι νυνὶ γέγονεν ἐκ τοῦ διαλόγου μηδὲν εἰδέναι· ὁπότε γὰρ τὸ δίκαιον μὴ οἶδα ὅ ἐστιν, σχολῇ εἴσομαι εἴτε ἀρετή τις οὖσα τυγχάνει εἴτε καὶ οὔ, καὶ πότερον ὁ ἔχων αὐτὸ οὐκ εὐδαίμων ἐστὶν ἢ εὐδαίμων.

Hence the result of the discussion, as far as I'm concerned, is that I know nothing, for when I don't know what justice is, I'll hardly know whether it is a kind of virtue or not, or whether a person who has it is happy or unhappy.

Republic, 354b-c (conclusion of book I), as translated by M.A. Grube in Republic (Grube Edition) (1992) revised by C.D.C. Reeve, p. 31

Confer Apology 21d (see above), Theaetetus 161b (see above) and Meno 80d1-3: "So now I do not know what virtue is; perhaps you knew before you contacted me, but now you are certainly like one who does not know."

Confer Cicero, Academica, Book I, section 1 http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A2007.01.0032%3Abook%3D1: "ipse se nihil scire id unum sciat ("He himself thinks he knows one thing, that he knows nothing"). Often quoted as "scio me nihil scire" or "scio me nescire." A variant is found in von Kues, De visione Dei, XIII, 146 (Werke, Walter de Gruyter, 1967, p. 312): "...et hoc scio solum, quia scio me nescire... [I know alone, that (or because) I know, that I do not know]." In the modern era, the Latin quote was back-translated to Greek as "ἓν οἶδα ὅτι οὐδὲν οἶδα", hèn oîda hóti oudèn oîda).

Confer Diogenes Laertius, II.32 (see above)
Misattributed

The Man in the High Castle (1962)

Source: Sweethearts

"A'Rebours"
Lyrics and poetry

Source: Tender Is the Night

Source: The Monkey Grammarian (1974), Ch. 1
Context: The best thing to do will be to choose the path to Galta, traverse it again (invent it as I traverse it), and without realizing it, almost imperceptibly, go to the end — without being concerned about what “going to the end” means or what I meant when I wrote that phrase. At the very beginning of the journey, already far off the main highway, as I walked along the path that leads to Galta, past the little grove of banyan trees and the pools of foul stagnant water, through the Gateway fallen into ruins and into the main courtyard bordered by dilapidated houses, I also had no idea where I was going, and was not concerned about it. I wasn’t asking myself questions: I was walking, merely walking, with no fixed itinerary in mind. I was simply setting forth to meet … what? I didn’t know at the time, and I still don’t know. Perhaps that is why I wrote “going to the end”: in order to find out, in order to discover what there is after the end. A verbal trap; after the end there is nothing, since if there were something, the end would not be the end. Nonetheless, we are always setting forth to meet … even though we know that there is nothing, or no one, awaiting us. We go along, without a fixed itinerary, yet at the same time with an end (what end?) in mind, and with the aim of reaching the end. A search for the end, a dread of the end: the obverse and the reverse of the same act. Without this end that constantly eludes us we would not journey forth, nor would there be any paths. But the end is the refutation and the condemnation of the path: at the end the path dissolves, the meeting fades away to nothingness. And the end — it too fades away to nothingness.

Source: The Lady of the Rivers

Clip from YouTube https://www.youtube.com/watch?v=3h00oBDCODs
Allen Iverson's reaction to being benched by Sixers coach Chris Ford in April 2004.