A System of Logic -- A priori and a posteriori -- Abacus logic -- Abduction (logic) -- Abductive validation -- Academia Analitica -- Accuracy and precision -- Ad captandum -- Ad hoc hypothesis -- Ad hominem -- Affine logic -- Affirming the antecedent -- Affirming the consequent -- Algebraic logic -- Ambiguity -- Analysis -- Analysis (journal) -- Analytic reasoning -- Analytic–synthetic distinction -- Anangeon -- Anecdotal evidence -- Antecedent (logic) -- Antepredicament -- Anti-psychologism -- Antinomy -- Apophasis -- Appeal to probability -- Appeal to ridicule -- Archive for Mathematical Logic -- Arché -- Argument -- Argument by example -- Argument form -- Argument from authority -- Argument map -- Argumentation ethics -- Argumentation theory -- Argumentum ad baculum -- Argumentum e contrario -- Ariadne's thread (logic) -- Aristotelian logic -- Aristotle -- Association for Informal Logic and Critical Thinking -- Association for Logic, Language and Information -- Association for Symbolic Logic -- Attacking Faulty Reasoning -- Australasian Association for Logic -- Axiom -- Axiom independence -- Axiom of reducibility -- Axiomatic system -- Axiomatization --