Socratic Logic - Judgement

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.

Contradictions

In logic a contradiction is the objective, logical relation between two propositions that cannot both be true at the same time and in the same sense.  One true and one false, but both cannot be true at the same time.

Two propositions contradict each other  when the truth of one proposition necessarily means the falsehood of the other one.

This can happen when:

• the propositions have the same subject and predicate
• the propositions differ in both quantity and quality

The Square of Opposition and Contradictories

Insert square image...

Contraries cannot both be true, but they can both be false.

Subcontraries can both be true, but cannot both be false.

....