4.13 The most common, indeed the most dully commonplace, case of the Subject/Motive Shift is that in which an assertion is dismissed as false or an argument is discredited as unsound for no other and better reason than that it is made or presented by an interested party. Certainly it is right always to be alert to the possibility that assertions and arguments are being corrupted by the self-interests of the asserters and the arguers. And there is little need to warn most of us to be alert to the possibility that the statements and arguments of businesspersons, politicians, labor-union spokespersons, and public-relations officers may be corrupted by various forms of self-interest. Everyone, too, knows the cynic's definition of ambassadors as people sent abroad to lie for their countries.
4.14 It may very well be that as an ambassador, labor-union official, or public-relations officer, he or she is paid to say a certain thing or argue in a certain way. But that material interest by itself does not constitute good or even any grounds for concluding that those representatives' claims must be false and their arguments must be invalid. Those of us who do not belong to any of these three suspect classes often present sound arguments for conclusions that happen to be both true and to our advantage. The truth is no more necessarily disagreeable than it is necessarily agreeable.
(Antony Flew, How to Think Straight: An Introduction to Critical Reasoning, 2d ed. [Amherst, NY: Prometheus Books, 1998], 65-6)
Note from KBJ: Here is a good discussion of the circumstantial ad hominem fallacy.
Keith Burgess-Jackson, J.D., Ph.D. 