Socratic Logic - Reasoning

Laws of Logic

The following are the notes I have taken from Peter Kreeft's wonderful Socratic Logic textbook. I highly recommend you buy it. This is a book which deals with classical logic (as opposed to modern symbolic logic.) It is easily the best overall book on logic I have ever read and one of the few I have that are worth making notes from. I even have a small duotang with these notes in it I can carry around and reference on a regular basis.

Five main laws of logic:

A. Law of Non-contradiction

B. Law of Identity

C. Law of Excluded Middle

D. Principle of Sufficient Reason

E. Principle of Causality

You cannot think correctly without following the above five laws.

The first three were used by Aristotle to refute skeptics objections to syllogisms.

The last two are added by the German logician Liebniz.

Law of Non-contradiction

A cannot be both B and non-B at the same time and in the same sense.

"Nothing can both be and not-be"

Propositions cannot be both true and false.

Law of Identity

x  is x.

Whatever is x, is x.

Law of Excluded Middle

Either x or non-x.

Every proposition must be true or false.

Principle of Sufficient Reason

Everything that is has a sufficient reason why it is.

(Both why it exists and what it is.)

Principle of Causality

Everything that acts or changes has a reason or cause why it acts or changes.

Can also be stated:

Everything that begins to exist has a reason why it exists.

ie. Stuff just doesn't pop into existence.