Introduction to Logic Tools for Thought   Help Lessons Chapters Dictionary Tools abduction analogy and elimination and introduction antecedent biconditional biconditional elimination biconditional introduction completeness conclusion conjunct conjunction consequent Consistency Theorem contingency contrapositive converse deduction Deduction Theorem disjunct disjunction Equivalence Theorem falsifiability Fitch system implication implication elimination implication introduction induction instance inverse linear proof logical constant logical consistency logical entailment logical equivalence metavariable negation negation elimination negation introduction operator precedence or elimination or introduction premise proof proposition proposition constant propositional language propositional logic propositional sentence propositional vocabulary provability reiteration relational logic rule of inference satisfaction satisfiability schema sentence soundness structured proof target truth assignment truth table unsatisfiability Unsatisfiability Theorem validity