  In Justification Logic, in addition to the category of formulas,  there is a second category of justifications. Justifications are formal terms, built up from constants and variables using various operation symbols. Constants represent justifications for commonly  accepted truths—typically axioms. Variables denote unspecified  justifications. Different justification logics differ on which  operations are allowed (and also in other ways too). If \(t\) is a justification term and \(X\) is a formula,  \(t : X\) is a formula, and is intended to be read: