13.2 Derivability
13.2 Derivability Derivability is the formal relation between assumptions and conclusions. It tells us when a formula can be obtained by applying the rules of a proof system. We write: $$ \Gamma \vdash A $$ to mean that $A$ is derivable from the assumptions in $\Gamma$. Here $\Gamma$ is a collection of formulas. It may contain one formula, many formulas, or no formulas at all. For example: $$ p,\ p...