Also called sentence logic and the sentential calculus. Such a logic concerns elementary propositions - p, q, r, s, etc. -- respecting which the only assumption is that they should individually be either true or false, and operators that form complex propositions when joined with appropriate numbers of elementary propositions. The operators include conjunction (&) hence ’p and q’; disjunction (v), hence ’p or q’; negation (-), hence ’-p’; conditional (--> ), hence ’If p then q’; and equivalence ( =), hence ’p is equivalent to q’. This logic is concerned with determining which complex propositions are logical truths, or tautologies; this effectively determines what are valid arguments because such can always be treated as complex propositions in which the premisses of the argument appear as the antecedent and the conclusion as the consequence. This logic, as opposed to first, or higher, order predicate logic is complete and decidable.