Hypothetical syllogisms (conditional arguments)
Posted by htonibutter on 04/27/2005 @ 21:23:48 PST

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Hypothetical syllogisms (conditional arguments) can have
two valid and two invalid structures. The two valid
structures are affirming the antecedent (modus ponens) and
denying the consequent (modus tollens). The two invalid
structures, or fallacies, are denying the antecedent and
affirming the consequent. Determine if the arguments below
are valid or invalid and what form they take. The first one
has been done for you.
(NOTE: The term "modus pollens" has been used in some texts
as a version of "modus ponens." For the purposes of this
exercise, "modus ponens" will be used.)

SAMPLE: Chris and Nick would be very happy if the Browns
beat the Steelers. The Browns did beat the Steelers, so
Chris and Nick are very happy, indeed.

Valid Modus Ponens (Affirming the Antecedent)

11. Madeline must have known the material for the test,
because if a person knows the material, that person will
always get an A, and Madeline was one of the students that
got an A.

12. Anastasia believes that if she treats people honestly
and with an open mind, she will have diverse friendships.
She is honest and open-minded and has friends all over the

13. Roberto thought that if he worked very hard, his boss
would give him a raise or a promotion. He made sure she
noticed him, but she did not give him a raise or promotion.
He thought he must not have worked hard enough.

14. "I'm still eating too much ice cream," lamented
George. "My waist measurement is the same as it was six
months ago. I know if I didn't eat so much ice cream, I
would reduce my waist size."

15. The good way to make sure we pay fewer taxes is to
elect conservatives. We must not have elected enough
conservatives, because we are paying more taxes.

16. If my neighbor were a decent human being, he wouldn't
let his tree leaves fall onto my property. But he's not a
decent human being, so we get to clean his tree leaves as
well as ours.

17. We'll have fun and learn a lot in this course, if we
have great facilitator. We do have a great facilitator;
therefore we are learning a lot and having fun doing it!

18. If I could only understand the concept of
hypothetical syllogisms, I would get an A in class. Hurray!
I got an A in class, so I must have understood the concept
of hypothetical syllogisms.

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