Every man carries about him a touchstone, if he will make use of it, to distinguish substantial gold from superficial glitterings, truth from appearances. And indeed the use and benefit of this touchstone, which is natural reason, is spoiled and lost only by assuming prejudices, overweening presumption, and narrowing our minds.

The Conduct of the Understanding; By John Locke... Essays, Moral, Economical, and Political; by Francis Bacon (ed. 1859)