View Full Version : Some examples of Deductive Reasoning

06-30-2009, 04:02 PM
There are rules for Deductive Reasoning. These are known as Valid arguments. If the premises are true, the conclusions will have to be true as well, based on the form of the argument.

Modes Ponens (Affirming):

If P then Q. P. Therefore, Q.

Ex. If there is smoke, something is burning. There is smoke, therefore, something is burning.

Modes Tollens (Denying):

If P then Q. Not P. Therefore, Not Q.

Ex. If you spill the water, the floor will be wet. The floor is not wet. Therefore, you did not spill the water.


If P then Q. If Q then R. P. Therefore, R.

Ex. "If you spill the water, then the floor will be wet. If the floor is wet, then it needs to be mopped up. You spilled the water, so the floor needs to be mopped up."

Valid Disjunctive Syllogism:

P or Q. Not P. Therefore, Q. (or P or Q. Not Q. Therefore P.)

Ex. It's Monday or Tuesday. It's not Monday. Therefore, it is Tuesday. (or) It's Monday or Tuesday. It's not Tuesday. Therefore, it is Monday. Disjunctive syllogisms are dependent on the "or" being exclusive. In other words, it must be one or the other, but cannot be both. If the "or" is inclusive, that is meaning one, the other, or both, this becomes a
logical fallacy.

These are a few of the basic valid arguments. Basically, when faced with an argument that takes a form you are not familiar with, break it down into its parts. (P, Q, R, etc.) Then, substitute something silly, like dogs, cats, penguins, and so on into those slots. If the argument becomes ridiculous, it's a fallacy.