“The work of science is to substitute facts for appearances, and demonstrations for impressions.”

Source: A Dictionary of Thoughts, 1891, p. 165.

Conclusion of his Nobel lecture http://nobelprize.org/nobel_prizes/physics/laureates/1978/wilson-lecture.html (December 8, 1978) emphasizing that every new experimental discovery increases significantly our knowledge.

Source: 1910s, Mysticism and Logic and Other Essays http://archive.org/stream/mysticism00russuoft/mysticism00russuoft_djvu.txt (1918), Ch. 1: Mysticism and Logic

Context: The beginnings of science have often the appearance of chance. A felicitous accident throws a certain natural fact under the notice of an inquiring and philosophic mind. Attention is awakened and investigation provoked. Similar phenomena under varied circumstances are eagerly sought for; and if in the natural course of events they do not present themselves, circumstances are designedly arranged so as to bring about their production. The seeds of science are thus sown, and soon begin to germinate.

"Body Pleasure and the Origins of Violence" (1975)

Doctorate Award Speech, Kean University (2004)

In a panel discussion on Real Time with Bill Maher, 02/08/2013

Introductory
A Treatise on Man and the Development of His Faculties (1842)
Context: It is a remarkable fact in the history of science, that the more extended human knowledge has become, the more limited human power, in that respect, has constantly appeared. This globe, of which man imagines the haughty possessor, becomes, in the eyes of astronomer, merely a grain of dust floating in immensity of space: an earthquake, a tempest, an inundation, may destroy in an instant an entire people, or ruin the labours of twenty ages.... But if each step in the career of science thus gradually diminishes his importance, his pride has a compensation in the greater idea of his intellectual power, by which he has been enabled to perceive those laws which seem to be, by their nature, placed for ever beyond his grasp.