Take the thoughts of such an one, used for many years to one tract, out of that narrow compass he has been all his life confined to, you will find him no more capable of reasoning than almost a perfect natural. Some one or two rules on which their conclusions immediately depend you will find in most men have governed all their thoughts; these, true or false, have been the maxims they have been guided by. Take these from them, and they are perfectly at a loss, their compass and polestar then are gone and their understanding is perfectly at a nonplus; and therefore they either immediately return to their old maxims again as the foundations of all truth to them, notwithstanding all that can be said to show their weakness, or, if they give them up to their reasons, they with them give up all truth and further enquiry and think there is no such thing as certainty.

-- John Locke, Of the Conduct of Understanding