“The social organization of work is the most complicated and difficult problem that humanity has ever had to solve. It being possible to realize that organization (a difficult translation or me: "Cette organisation ne pouvant être réalisée par", Fr.) neither by violence, nor by merely external (or outward) or legal measures, it require the free participation of all (or everybody) to the joint (or common, or in commun) work, and, consequently, to a regeneration (or reconditioning) of men that bring them to overcome their selfishness et understand their duty towards themselves et towards the community.”

1950s, Three Ways of Meeting Oppression (1958)
Source: A Testament of Hope: The Essential Writings and Speeches
Context: A second way that oppressed people sometimes deal with oppression is to resort to physical violence and corroding hatred. Violence often brings about momentary results. Nations have frequently won their independence in battle. But in spite of temporary victories, violence never brings permanent peace. It solves no social problem; it merely creates new and more complicated ones.

Source: Sweet Thursday (1954), Ch. 20

Source: The Nature of Geography (1939), p. 216-217

Source: The transformation of American industrial relations, 1986, p. 147

Sixth Thesis
Idea for a Universal History from a Cosmopolitan Point of View (1784)

"Simpler Social Problems Have Proved Intractable", paragraph 138
Industrial Society and Its Future (1995)

Source: New patterns of management, (1961), p. 5

Source: Social Problems (1883), Ch. 1 : The Increasing Importance of Social Questions
Context: The intelligence required for the solving of social problems is not a thing of the mere intellect. It must be animated with the religious sentiment and warm with sympathy for human suffering. It must stretch out beyond self-interest, whether it be the self-interest of the few or of the many. It must seek justice. For at the bottom of every social problem we will find a social wrong.

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.