“That fellow seems to me to possess but one idea, and that is a wrong one.”

Variants (these could be paraphrases or differing translations): The belief that there is only one truth and that oneself is in possession of it seems to me the deepest root of all evil that is in the world.
The belief that there is only one truth, and that oneself is in possession of it, is the root of all evil in the world.
Source: Natural Philosophy of Cause and Chance (1964), p. 230, also in My Life and Views (1968), p. 183

Source: Kindred

As quoted in "Cary Grant is puzzled because you have No Time for Laughs" by Robert Ottaway in Picturegoer magazine (4 January 1958)

Bk. 1, ch. 1
The End of the Affair (1951)

p. 29 https://books.google.com/books?id=GHkIAAAAIAAJ&pg=PA29
Pioneer Work in Opening the Medical Profession to Women (1895)

Nobel lecture (2001)
Context: The idea that there is one people in possession of the truth, one answer to the world’s ills, or one solution to humanity’s needs, has done untold harm throughout history — especially in the last century. Today, however, even amidst continuing ethnic conflict around the world, there is a growing understanding that human diversity is both the reality that makes dialogue necessary, and the very basis for that dialogue.
We understand, as never before, that each of us is fully worthy of the respect and dignity essential to our common humanity. We recognize that we are the products of many cultures, traditions and memories; that mutual respect allows us to study and learn from other cultures; and that we gain strength by combining the foreign with the familiar.

Source: On Photography

Source: Private Domain: An Autobiography