Re: modus ponens & modus tollens?

[Maxwell Gwynn]

Tim:

In a nutshell:
According to Ray (2000), modus ponens involves confirmatory reasoning: 
If p, then q. 
P. 
Therefore, q. 

If I read it on TIPS, it must be right. 
I read it on TIPS. 
Therefore, it must be right. 

Modus Tollens involves disconfirmatory reasoning: 
If p, then q. 
Not q. 
Therefore, not p.
If I read it on TIPS, it must be right. 
It is not right. 
Therefore, I did not read it on TIPS.

Although both forms are logically valid, both depend on the truth of the
statement (or the theory, in your case)
that if I read it on TIPS, it must be right. We can't confirm the theory
nor the antecedent (that I in fact read it on TIPS) using either of these
two forms of syllogistic reasoning. 

I don't know the etiology of the terms, but you can check out: 

http://www.bu.edu/wcp/Papers/Logi/LogiDagl.htm

which states:

According to the Stoic logicians, the first kind of indemonstrable
statements is as follows: "If the first, then the second; but the first;
therefore the second." We call this basic argument form as modus ponendo
ponens, in abbreviation modus ponens, the mood that by affirming affirms.
The second kind of indemonstrable statements of the Stoics is: "If the
first, then the second; but the second is not; therefore the first is
not." This basic argument form is called as modus tollendo tollens, in
abbreviation modus tollens, the mood that by denying denies, nowadays.(2)


-Max


On Fri, 26 Jan 2001, Tim Gaines wrote:

> I'm putting TIPS to the test this morning.  In 30 minutes I'll
> be talking to my Research Methods class about the logic of
> testing theories, and one point I want to make is that neither
> of the two logically valid modes of syllogistic reasoning
> (modus ponens and modus tollens) permits confirmation of the
> antecedent (the theory).  It would be nice to be able to
> provide the etiology of the terms, but I have loaned my copy
> of Wason and Johnson-Laird to a student, and I can't seem to
> find the terms in other references I have.  Anyone know these
> terms?
> 
> Thanks,
> 
> Tim
> -- 
> 
> ****************************************************************
>   Tim Gaines                          [EMAIL PROTECTED]
>   Professor of Psychology             phone:  864-833-8349
>   Presbyterian College                        fax:  864-833-8481
>   Clinton, SC  29325
> ****************************************************************
> 

Maxwell Gwynn, PhD                              [EMAIL PROTECTED]
Department of Psychology                        (519) 884-0710 ext 3854
Wilfrid Laurier University
Waterloo, Ontario  N2L 3C5 Canada