# Definition:Classical Propositional Logic

## Definition

Classical propositional logic is the branch of propositional logic based upon Aristotelian logic, whose philosophical tenets state that:

$(1): \quad$ A statement must be either true or false, and there can be no middle value.
$(2): \quad$ A statement cannot be both true and not true at the same time, that is, it may not contradict itself.

Thus, we proceed by recalling the formal language of propositional logic $\LL_0$.

To make $\LL_0$ into a formal system, we need to endow it with a deductive apparatus.

That is, with axioms and rules of inference.

There are several (equivalent) methods of defining such a deductive apparatus.

One popular and particularly elegant set of axioms for classical logic was devised by Jan Łukasiewicz. It consists of three axioms and an inference rule: modus ponens. The axioms are as follows:

$\vdash \phi \implies \paren {\psi \implies \phi}$
$\vdash \paren {\phi \implies \paren {\psi \implies \chi} } \implies \paren {\paren {\phi \implies \psi} \implies \paren {\phi \implies \chi} }$
$\vdash \paren {\not \psi \implies \not \phi} \implies \paren {\phi \implies \psi}$