Deductive Logic (first blog...YAY!)

We all use deductive reasoning on a daily basis, but we may not be aware that we are using it. The three main types of deductive logic are: syllogisms, modus ponens, and modus tollens. Personally, I find syllogisms to be the hardest to understand and test for validity and/or truth. Modus ponens and modus tollens are much easier for me to distinguish. Modus ponens state:

If p, then q

p

Therefore, q

Modus tollens, on the other hand, are the negative of modus ponens, so they state:

If p, then q

not q

Therefore, not p

Here is an example of how they differ:

Modus Ponen:

If Emily is a buttface, then I will stick my tongue out at her.

Emily is a buttface.

Therefore I will stick my tongue out at her.

VERSUS

Modus Tollen

If Emily is a buttface, then I will stick my tongue out at her.

I will not stick out my tongue at her

Therefore, she is not a buttface. 

In a modus ponen, the q term is being concluded, but in a modus tollen, the the p term is being concluded. While they both start with a statement including both the p and q terms, their deductive logic and hence their conclusions take different paths.