Deduction

A deduction in formal logic is a way of proving a proposition. It is used most often in geometry, but also has its place in philosophy and law. Sadly, it is often left out of ideology and even science.

When a conclusion is inferred from premises or facts, it is said to "follow" from previously stated propositions via certain logical rules.

The most famous rule goes as follows:

1. A is true.
2. If A is true, then B is true.
3. Therefore, B is true.

A lesser known rule is:

1. If Q is true, then R is true.
2. R is not true.
3. Therefore, Q is not true.

Aristotle codified the rules of deduction two millenia ago in Ancient Greece. He also added 256 rules about groups of things. For example:

1. All men are mortal.
2. Socrates is a man.
3. Therefore, Socrates is mortal.

See also:

• induction