“The problem is, eternity is barred to humans, and so humans, all too painfully aware of that and entertaining little hope of appealing against that verdict of fate, seek to stifle and deafen their tragic wisdom in a hubbub of frail and fleeting pleasures. This admittedly being a false calculation—for the same reason which prompted it (that tragic wisdom can never be chased or conjured away for good)—they condemn themselves, whatever their material wealth, to perpetual spiritual poverty: to continuous unhappiness (‘A man is as unhappy as he has convinced himself to be’). Instead of seeking the way to happiness within the limits of their predicament, they take a long detour, hoping that somewhere along the route their odious and repulsive destiny may be escaped or fooled—only to land back in the despair that prompted them to start on their voyage of (dearly wished for, yet unattainable) discovery. The only discovery humans can possibly make on that voyage is that the route they have taken was but a detour that sooner or later will bring them back to the starting line.”

Source: Robots and Empire

As it is: The consolation of action (p. 193)
Straw Dogs: Thoughts on Humans and Other Animals (2002)

Also: Until the day when God shall deign to reveal the future to man, all human wisdom is summed up in these two words,— "Wait and hope".
Chapter 117 http://en.wikisource.org/wiki/The_Count_of_Monte_Cristo/Chapter_117
Variant: All human wisdom is contained in these two words - Wait and Hope
Source: The Count of Monte Cristo (1845–1846)

Source: The Second Light (1986), p. 14

Prospects on the Rubicon http://books.google.com/books?id=PN9bAAAAQAAJ&q=%22War+involves+in+its+progress+such+a+train+of+unforseen+and+unsupposed+circumstances%22+%22that+no+human+wisdom+can+calculate+the+end%22&pg=PA5#v=onepage (1787).
1780s

The Great Indian Novel
Variant: A philosopher is a lover of wisdom, not of knowledge, which for all its great uses ultimately suffers from the crippling effect of ephemerality. All knowledge is transient linked to the world around it and subject to change as the world changes, whereas wisdom, true wisdom is eternal immutable. To be philosophical one must love wisdom for its own sake, accept its permanent validity and yet its perpetual irrelevance. It is the fate of the wise to understand the process of history and yet never to shape it.

Letter to Nathaniel Hawthorne, including bits of a review of his work that he had written (c. 16 April 1851); published in Nathaniel Hawthorne and His WIfe Vol, I (1884) by Julian Hawthorne, Ch. VIII : Lenox, p. 388
Context: There is a certain tragic phase of humanity which, in our opinion, was never more powerfully embodied than by Hawthorne. We mean the tragedies of human thought in its own unbiassed, native, and profounder workings. We think that into no recorded mind has the intense feeling of the usable truth ever entered more deeply than into this man's. By usable truth, we mean the apprehension of the absolute condition of present things as they strike the eye of the man who fears them not, though they do their worst to him, — the man who, like Russia or the British Empire, declares himself a sovereign nature (in himself) amid the powers of heaven, hell, and earth. He may perish; but so long as he exists he insists upon treating with all Powers upon an equal basis. If any of those other Powers choose to withhold certain secrets, let them; that does not impair my sovereignty in myself; that does not make me tributary. And perhaps, after all, there is no secret. We incline to think that the Problem of the Universe is like the Freemason's mighty secret, so terrible to all children. It turns out, at last, to consist in a triangle, a mallet, and an apron, — nothing more! We incline to think that God cannot explain His own secrets, and that He would like a little information upon certain points Himself. We mortals astonish Him as much as He us. But it is this Being of the matter; there lies the knot with which we choke ourselves. As soon as you say Me, a God, a Nature, so soon you jump off from your stool and hang from the beam. Yes, that word is the hangman. Take God out of the dictionary, and you would have Him in the street.
There is the grand truth about Nathaniel Hawthorne. He says NO! in thunder; but the Devil himself cannot make him say yes. For all men who say yes, lie; and all men who say no,—why, they are in the happy condition of judicious, unincumbered travellers in Europe; they cross the frontiers into Eternity with nothing but a carpet-bag, — that is to say, the Ego. Whereas those yes-gentry, they travel with heaps of baggage, and, damn them! they will never get through the Custom House. What's the reason, Mr. Hawthorne, that in the last stages of metaphysics a fellow always falls to swearing so? I could rip an hour.

Source: Stranger in a Strange Land