Formal logic (or symbolic logic) is the Western Civilization's system of reasoning. It is used in law, religion, science, and mathematics.
Logic is used in proofs. For example, given that
- All men are mortal; and,
- Socrates is a man;
we can deduce (and prove) that
- Socrates is mortal.
Using one of the same premises, we can prove that Lucifer is not a man:
- Lucifer is immortal (i.e., not mortal)
- Whoever is not mortal, is not a man.
- Hence, Lucifer is not a man.
The key element here is the inference from All men are mortal to whoever is not mortal is not a man (see Syllogisms)
Rules of inference
- If A, then B.
- Therefore, B.
- If not A, then not B.
- Not B
- Therefore, not A.