Advances in knowledge are not commonly made without the previous exercise of some boldness and license in guessing. The discovery of new truths requires, undoubtedly, minds careful and fertile in examining what is suggested; but it requires, no less, such as are quick and fertile in suggesting.

History of the Inductive Sciences, from the Earliest to the Present Time - (Volume 1), Book V, Chapter IV, Section 1 (p. 411)