'Facts, facts, facts,' cries the scientist if he wants to emphasize the necessity of a firm foundation for science. What is a fact? A fact is a thought that is true. But the scientist will surely not recognize something which depends on men's varying states of mind to be the firm foundation of science.

Gottlob Frege (1956). "The thought: A logical inquiry" in: Peter Ludlow (1997) Readings in the Philosophy of Language. p. 27