To explain all nature is too difficult a task for any one man or even for any one age. 'Tis much better to do a little with certainty, & leave the rest for others that come after you, than to explain all things by conjecture without making sure of any thing.
Statement from unpublished notes for the Preface to Opticks (1704) quoted in Never at Rest: A Biography of Isaac Newton (1983) by Richard S. Westfall, p. 643