Saturday, November 19, 2005

Logic and Arguments

I just wanted to give a quick primer on logic and arguments:

Generally, an argument is logical if it follows the laws of logic, which typically take standard forms: modus ponens, modus tollens, dysjunctive syllogism, etc.

Any argument that follows these rules is considered valid. If it violates these forms, it is invalid.

Example of modus ponens:
1. If it is raining outside, then the streets are wet. (P--> Q)
2. It is raining outside. (P)
3. Therefore, the streets are wet. (Q)

This is a valid argument. The following is also valid:

1. If the moon is made of green cheese, then I am the president. (P--> Q)
2. The moon is made of green cheese. (P)
3. Therefore, I am the president. (Q)

While clearly false, this is a valid argument because it follows the rules of logic.
If an argument is valid and its premises are true, then the argument is sound. The first argument is sound (assuming it is raining outside), while the second is unsound.

So, if someone claims that a particular argument is fallacious or illogical, he must show how it violates the rules and laws of logic, OR he must show that one of its premises is false. For instance, the second argument above is valid, but someone could defeat it simply by showing that the moon is not, in fact, made of green cheese. If premise (2) is false, then the argument is unsound.


Blogger Blog World said...

Faith is spiritualized imagination.
Henry Ward Beecher- Posters.

10:42 AM  
Anonymous Christian said...

chris, I was just passing by on my search for things about Church on
the Net, and dropped in on your blog. I was looking for stuff for my Church site. Not
sure that your blog was exactly what I needed, but I enjoyed my visit all the same.

2:09 AM  
Blogger Jeff Pioquinto,SJ said...

good point. thanks

12:48 PM  

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