“If the mind is to emerge unscathed from this relentless struggle with the unforeseen, two qualities are indispensable: first, an intellect that, even in the darkest hour, retains some glimmerings of the inner light which leads to truth; and second, the courage to follow this faint light wherever it may lead.”

Introductory : The Problem
Progress and Poverty (1879)
Context: I propose in this inquiry to take nothing for granted, but to bring even accepted theories to the test of first principles, and should they not stand the test, freshly to interrogate facts in the endeavor to discover their law.
I propose to beg no question, to shrink from no conclusion, but to follow truth wherever it may lead. Upon us is the responsibility of seeking the law, for in the very heart of our civilization to-day women faint and little children moan. But what that law may prove to be is not our affair. If the conclusions that we reach run counter to our prejudices, let us not flinch; if they challenge institutions that have long been deemed wise and natural, let us not turn back.

Letter https://founders.archives.gov/documents/Jefferson/98-01-02-1712 to William Roscoe (27 December 1820)
1820s

Source: On Liberty

"The Commitment of the Intellectual," in The Longer View (1969), p. 14

The Golden Legend, Pt. V, A Covered Bridge at Lucerne.

1910s, Speech in the Reichstag, 18 March 1918

Phaedrus
Plato, Phaedrus

Speech at the University of Wisconsin, Madison (8 October 1952)

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 264.