Logic Lesson: Disjunctive Syllogisms

Thank God it’s Friday! That way John won’t order this 6 pound cheeseburger if he goes out tonight.

But if it were, say, Tuesday, the conversation might go like this:

“Either that’s the biggest cheeseburger I’ve ever seen, or I’m shrinking precipitously!” John exclaims.
“You’re not shrinking precipitously,” replies John’s wife.
“I guess that’s the biggest cheeseburger I’ve ever seen!” John says, now salivating precipitously.

The components of John’s disjunctive proposition – biggest cheeseburger he’s ever seen or he’s shrinking precipitously – are called disjuncts. So long as we eliminate all the disjuncts but one, that one must be true – assuming, of course, that the disjunctive premise is true to begin with.

Here it is in standard form:

Either that’s the biggest cheeseburger John’s ever seen, or he’s shrinking precipitously!
He’s not shrinking precipitously.
That’s the biggest cheeseburger John’s ever seen.

Here’s another example.

Either John Kerry was in Cambodia on Christmas in 1968 or he’s a big liar and shouldn’t be elected President.
John Kerry was not in Cambodia on Christmas in 1968.
He’s a big liar and shouldn’t be elected President.

8 comments

  1. Of course it’s possible. Not advisable, but possible.
    …meanwhile, and you knew I had to do it, is there ever going to be a post where John Kerry isn’t mentioned in some way? I thought for sure this one was safe, but lo and behold.

  2. …meanwhile, and you knew I had to do it, is there ever going to be a post where John Kerry isn’t mentioned in some way?
    Huh? I see tons of posts on the front page alone without Kerry mentioned in any way.

  3. On the front page, 6 out of 23 posts mention sKerry in some way or another. Thats what, barely over one quarter?

  4. No, but I did once eat the 96oz steak, plus three giant baked potatoes loaded to the brim with cheese, sour cream and bacon, at The Possum Trot in Decatur, AL.
    I bet I could have finished the burger when I was 14. Now I have much better sense.

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