[N]obody can be compelled to assent to the conclusion of even a valid inference unless he accepts all its premisses.
(R. M. Hare, "Wanting: Some Pitfalls," chap. 3 in Practical Inferences [Berkeley and Los Angeles: University of California Press, 1972], 44-58, at 50 [essay first published in 1971])
Note from KBJ: Argumentation, like criminal law, is coercive. The arguer says to the interlocutor, in effect, "If you accept these premises, then you must accept the conclusion." Why? Because, in a valid deductive argument, the truth of the premises is incompatible with the falsity of the conclusion. Note the "if." It is always open to the interlocutor to reject one or more of the arguer's premises. Sometimes philosophers say that rejecting a premise is irrational. This is evasive. The point of argumentation is to persuade. You cannot do this (as Hare says) unless your interlocutor accepts your premises. This is why argumentation is difficult. First, you must know your audience. If I know that you are an act utilitarian, for example, then I may use act utilitarianism as one of my premises, in an attempt to persuade you to accept my conclusion. It would be pointless of me to use act utilitarianism as a premise if I were trying to persuade a deontologist, such as a Kantian. That person would simply reject my major premise, leaving things as they were. Second, your argument must be valid. The premises must really entail the conclusion, not just appear to. Moreover, your interlocutor must accept the validity of your argument. It's tempting to say that argumentation is a search for common ground. That's not so. I don't need to be an act utilitarian in order to use act utilitarianism as a premise in my argument. My interlocutor needs to be an act utilitarian. Knowing your audience is the first and most important step toward being an effective arguer.
Note 2 from KBJ: Suppose you reject one of the premises of my argument. I need not give up at that point. I may construct a new argument that has that proposition as its conclusion. My aim is to get you to accept what you previously rejected. Suppose you reject a premise of this new argument. I may construct a third argument that has that proposition as its conclusion. This process can go on indefinitely, although it requires a great deal of patience and good will on the part of the participants.
Keith Burgess-Jackson, J.D., Ph.D. 