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Think about the following situation.You know that
at least one of two alternatives (call them "P" and "Q")
is true. (We write this as "P v
Q" of course.) Now imagine that you find out that one of those
alternatives is actually false. What does that say about the
other one? Well if P is false, and P v
Q is true, it follows that Q must be true. (Otherwise P v
Q would be false too.) Likewise, if Q turned out to be false, P would
then have to be true. This gives us a rule:
Rule 7. Disjunctive Syllogism (Abbreviated by "DS")

And here's disjunctive syllogism in action:
Now, here's a series of arguments (which I thought
were all vaild, but which aren't) of steadily increasing complexity.
See how many of the valid ones you can derive!
No,
seriously, try to derive these arguments. (There's going to be a
quiz, you know!)
If you get through with those, see which of
the following are invalid, which are valid, which valid ones can be
derived using DS, and which can't.


Copyright © 2009 by
Martin C. Young
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