Propositional Logic

The 9 Rules of Propositional Logic

Propositional logic is the most basic level of logic, dealing with inferences based on sentential connectives, such as "if...., then", "or", and "and". There are 9 rules of inference. Equipped with these, you will be able to assess the validity of most of the arguments you will ever here. Remember that a good deductive argument is one which:

1. Is formally and informally valid (i.e the conclusion(s) must follow from the premises in accord with the rules of logic)
2. Has premises which are more plausible than their negation

I'm going to post each of the 9 rules here in 9 separate posts, with examples and perhaps a short discussion on each. I'll start with the first, "modus ponens". Knowing these rules is vital in the task of christian apologetics. First we must know that our arguments and reasons for believing in the existence of god and the christian gospel are reasonable. Secondly, we must be able to point out any logical inconsistencies to refute and correct others (with gentleness and respect - 1 Peter 3:15)

Rule #1: modus ponens

1. P -> Q
2. P
3. Q

Examples

1. If John studies hard, he will get a good grade in logic
2. John studies hard
3. John will get a good grade in logic

1. If it is a Sunday, the library is closed
2. It is a Sunday
3. The library is closed