Suppose, just to simplify matters, that there are only two political positions: call them Left and Right. Suppose you are an adherent of one of these positions—let us say the Left. Your position, that of the Left, will be a [sic] amalgam of various theses. For example: That people are, in general, better off when the Left party is in power than when the Right party is in power. That the policies of the Right party, although publicly defended by appeal to abstract principles, in reality reflect the interests of large multi-national corporations. That members of the political Left are, in general, more intelligent, better educated, and more compassionate than members of the political Right. That capital punishment is an ineffective deterrent to murder. That a three-month old fetus is not a human being. That there would be less use of hard drugs if they were legal and under strict control. And so on and so on. (In giving these examples of theses of which the political position of the Left is an amalgam, I have deliberately not used moral terms like ‘should’ or ‘unfair’, despite the fact that most people will immediately begin to talk in moral terms if you ask them to describe their politics. I have avoided these terms because I wanted to present what are uncontroversially a set of theses, and many philosophers believe that statements couched in moral terms, statements like ‘The state should, before all other things, ensure the fair distribution of goods and services’, are not properly speaking theses, but rather the expressions of attitudes.) Now do you think, as a member of the Left, that there are reasons that support these theses to which you subscribe, reasons that decisively outweigh the reasons the members of the Right could adduce in support of their own—of course contrary—theses? If you really are an adherent of the Left, it is very probable that you do. But if you spend any time arguing with someone of your own level of intelligence and education on the Right, you will find that the exchange of arguments bears a disquieting resemblance to the arguments of the philosophers about free will or materialism or the objectivity of morality. That is, neither of you will convince the other: the reasons that seem to you to be decisive arguments in favor of your position will seem to the other to be answerable and he or she will be happy to give you the answers. Of course, you won’t think that these answers are effective answers to your arguments, but it will be quite plain that your opponent will think they are. In fact, if both of you have any capacity for abstract thought, you will see that what you are arguing about is just exactly philosophy—political philosophy. And if you have any capacity for self-doubt—a very big if indeed—you will begin to wonder whether the arguments you can advance in favor of the theses you hold really do decisively outweigh the arguments that your colleague on the Right has advanced for the contrary positions.
(Peter van Inwagen, "Can Science Disprove the Existence of God?" Philosophic Exchange 34 [2004]: 39-53, at 51-2 [italics in original])
