Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!gatech!bloom-beacon!tjpak From: tjpak@athena.mit.edu (Tae J Pak) Newsgroups: sci.philosophy.tech Subject: Modus Ponens = NIL ? Message-ID: <1122@bloom-beacon.MIT.EDU> Date: Sun, 12-Jul-87 11:15:33 EDT Article-I.D.: bloom-be.1122 Posted: Sun Jul 12 11:15:33 1987 Date-Received: Mon, 13-Jul-87 01:18:54 EDT References: <3587e521.44e6@apollo.uucp> <680@gargoyle.UChicago.EDU> Sender: daemon@bloom-beacon.MIT.EDU Reply-To: tjpak@speaker.UUCP (Tae J Pak) Distribution: world Organization: Massachusetts Institute of Technology Lines: 49 Keywords: logic theory belief truth consistency In article <19647@ucbvax.BERKELEY.EDU> kube@cogsci.berkeley.edu.UUCP (Paul Kube) writes: ] It's October 1980. You hold the following plausible beliefs: ] 1. If it's a Republican that will win the election, then if ] Reagan doesn't win, Anderson will. ] 2. It's a Republican that will win the election. ]However, you don't believe what follows from these by modus ponens, ]viz. that if Reagan doesn't win, Anderson will (everyone believed that ]if Reagan didn't win, Carter would). An article on this dilemma (and in fact, citing this very example) appeared in a recent issue of _Analysis_, a philosophical journal. I would suggest to those who are interested that they refer to this very fine journal. In the mean time, I would like to propose a solution of my own. Now it seems to me that there is a fundamental ambiguity in the two statements above. Consider Statment 2: "It's a Republican that will win the election." What is one to make of this? Is it a statement of _FACT_ (i.e. in the same category as, say, the Law of Universal Gravitation) or is it merely a _SENTIMENT_ (much like "Dem Bums will never win the Pennant!") ? It is crucial that this ambiguity be resolved, else it cannot be certain that Statement 2 is an affirmation of the IF clause of Statement 1. I find it highly implausible that Statement 2 is a statement of fact, so for the moment let us assume it is a sentiment. If that is the case then clearly Statement 2 does not affirm the IF clause of Statement 1, for Statement 1 assumes the factual interpretation and not the sentimental interpretation. That is to say, Statement 1 makes the claim that IF a Republican MUST win, then it will either be Reagan or Anderson. Further more, if in fact the IF clause of Statement 1 is assuming a sentimental interpretation, then I would claim Statement 1 is unreasonable. A much more reasonable statement would be: 1A: "If it's a Republican that will win the election, then if Reagan doesn't win, Carter will win." This statement is possible because now the (first) IF clause is no longer a statement of fact and, thus, our choices are no longer restricted to the Republican field. For all intents and purposes, we can disregard the initial IF clause because it has no bearing on the statements that follow. To sum up, one of the following three must be true: (a) Statment 2 and the initial IF clause of Statement 1 are both facts, in which case modus ponens works fine; (b) Statement 2 and the initial IF clause of Statement 1 are both sentiments, in which case I claim Statement 1 is incorrect and should be replaced by Statement 1A (and modus ponens again works); (c) Statement 2 and the initial IF clause of Statement 1 are of mixed type, in which case modus ponens doesn't apply. Comments? --Tony Pak tjpak@speaker.mit.edu