David Hume 
An Enquiry Concerning the Principles of Morals 

DISPUTES with men, pertinaciously obstinate in their principles, are, of all others, the most irksome; except, perhaps, those with persons, entirely disingenuous, who really do not believe the opinions they defend, but engage in the controversy, from affectation, from a spirit of opposition, or from a desire of showing wit and ingenuity, superior to the rest of mankind. The same blind adherence to their own arguments is to be expected in both; the same contempt of their antagonists; and the same passionate vehemence, in inforcing sophistry and falsehood. And as reasoning is not the source, whence either disputant derives his tenets; it is in vain to expect, that any logic, which speaks not to the affections, will ever engage him to embrace sounder principles.