## What is a Mathematical Logic?

A mathematical logic is a formal system of formulas (**axioms**) and
**rules** for deriving other formulas, called **theorems**.

A **proof** is a derivation of a theorem. To see a concrete proof
tree, click here.

Why should you care? The neat thing about Theorems is that they are
``true.'' More precisely, if all the axioms are valid and the rules
are validity preserving, then anything derived from the axioms via the
rules is valid.

So, if you want to determine if some formula is true, **prove it**.