“When we jump to a conclusion before doing research, we tend to look for evidence that supports our beliefs and ignore facts that contradict them. This deprives us of the opportunity to grow our knowledge.”

Variant translation: The more we learn about the world, and the deeper our learning, the more conscious, clear, and well-defined will be our knowledge of what we do not know, our knowledge of our ignorance. The main source of our ignorance lies in the fact that our knowledge can only be finite, while our ignorance must necessarily be infinite.
Conjectures and Refutations: The Growth of Scientific Knowledge (1963)
Context: The more we learn about the world, and the deeper our learning, the more conscious, specific, and articulate will be our knowledge of what we do not know, our knowledge of our ignorance. For this, indeed, is the main source of our ignorance — the fact that our knowledge can be only finite, while our ignorance must necessarily be infinite.

Memorials of a Tour in Italy (1837), IV ("story" refers to History).

Source: Womenfolks: Growing Up Down South (1983), p. 1 (opening lines)

Montreal Gazette, April 2, 2003: On the Iraq war.
2003

Introduction à l'Étude de la Médecine Expérimentale (1865)

Source: The Lost Symbol

as quoted by Gordon Shrum. In an article by Robert Craig Brown, The life of Sir John Cunningham McLennan http://www.physics.utoronto.ca/overview/history/mclennan, Physics in Canada, March / April 2000.

From a speech to the Lautoka Rotary Club, 13 May 2005