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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
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Reply-To: tjpak@speaker.UUCP (Tae J Pak)
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Organization: Massachusetts Institute of Technology
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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