“Every good mathematician is at least half a philosopher, and every good philosopher is at least half a mathematician.”

"The Sad Fortunes of the Rev. Amos Barton" Ch. 4
Scenes of Clerical Life (1858)

p, 125
Number: The Language of Science (1930)

I Want to be a Mathematician: An Automathography (1985)
Context: Mathematics is not a deductive science — that's a cliché. When you try to prove a theorem, you don't just list the hypotheses, and then start to reason. What you do is trial and error, experimentation, guesswork. You want to find out what the facts are, and what you do is in that respect similar to what a laboratory technician does. Possibly philosophers would look on us mathematicians the same way as we look on the technicians, if they dared.

In a letter to Heinrich Wilhelm Matthias Olbers (14 May 1826), defending Chevalier d'Angos against presumption of guilt (by Johann Franz Encke and others), of having falsely claimed to have discovered a comet in 1784; as quoted in Calculus Gems (1992) by George F. Simmons

Scene X, Earth's Surface
Festus (1839)

A Retrospective Glance at the Lifework of a Master of Books
Fragments of a Poetics of Fire (1988)

Upon hearing that one of his students had dropped out to study poetry, as quoted in [http://books.google.com/?id=nnpChqstvg0C&pg=PA151 The Universal Book of Mathematics (2004) by David J. Darling, p. 151

Source: The Wealth of Nations (1776), Book I, Chapter II, p. 17.
Context: By nature a philosopher is not in genius and disposition half so different from a street porter, as a mastiff is from a greyhound

Source: Every Day