“I was convinced in 1927 that humanity's most fundamental survival problems could never be solved by politics.”

A World Restored: Metternich, Castlereagh and the Problems of Peace, 1812-22 (1957), p. 206
Paraphrased variant: The most fundamental problem of politics is not the control of wickedness but the limitation of righteousness.
Quoted by Walter Isaacson, " Henry Kissinger Reminds Us Why Realism Matters http://time.com/3275385/henry-kissinger/", Time, 4 September 2014
1950s

Source: Confessions of a Philosopher (1997), p. 157
Context: As Voltaire once remarked, "It is the privilege of the real genius, especially one who opens up a new path, to make great mistakes with impunity." The Copernican revolution brought about by Kant was, I think, the most important single turning point in the history of philosophy. For that reason there has been, ever since, a watershed in understanding between those who have taken his work on board and those who have not. For a good many of the problems he uncovered, the solutions he put forward have not stood the test of time, but his uncovering of the problems remains the most illuminating thing a philosopher has ever done. Because of the fundamental character of these problems, and because Kant did not solve them, confronting them has been the most important challenge to philosophy ever since.

Nova Interview

"Writing Plays for Television" in New World Writing, #10 (1956)
1970s, Homage to Daniel Shays : Collected Essays (1972)

Introduction.
On the Complexity of Causal Models (1977)

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

Source: Wild: From Lost to Found on the Pacific Crest Trail