“[I]n this life-long fight, to be waged by everyone of us as single-handed against a host of foes, the last requisite for a good fight, the last proof and test of our courage and manfulness, must be loyalty to truth — the most rare and difficult of all human qualities. For such loyalty, as it grows in perfection, asks ever more and more of us, and sets before us a standard of manliness always rising higher and higher.”

Letter Seven (14 May 1904)
Letters to a Young Poet (1934)
Variant: For one human being to love another human being: that is perhaps the most difficult task that has been given to us, the ultimate, the final problem and proof, the work for which all other work is merely preparation.
Source: The Selected Poetry of Rainer Maria Rilke
Context: People have (with the help of conventions) oriented all their solutions toward the easy and toward the easiest side of the easy; but it is clear that we must hold to what is difficult; everything alive holds to it, everything in Nature grows and defends itself in its own way and is characteristically and spontaneously itself, seeks at all costs to be so and against all opposition. We know little, but that we must hold to what is difficult is a certainty that will not forsake us; it is good to be solitary, for solitude is difficult; that something is difficult must be a reason the more for us to do it.
To love is good, too: love being difficult. For one human being to love another: that is perhaps the most difficult of all our tasks, the ultimate, the last test and proof, the work for which all other work is but preparation.

Speech on Leadership in Speeches Delivered on Various Occasions, May 1957-December 1959 (1960), p. 138.
Context: The art of leadership is in the ability to make people want to work for you, while they are really under no obligation to do so. Leaders are people, who raise the standards by which they judge themselves and by which they are willing to be judged. The goal chosen, the objective selected, the requirements imposed, are not mainly for their followers alone.
They develop with consumate energy and devotion, their own skill and knowledge in order to reach the standard they themselves have set.
This whole-hearted acceptance of the demands imposed by even higher standards is the basis of all human progress. A love of higher quality, we must remember, is essential in a leader.

Paris Review interview (1958)

God Knows (1984)

Source: As You Like It

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 395.

Letter to his wife http://members.tripod.com/iwo_jima_project/id6.htm.

Source: Vamps and Tramps (1994), p. xix