2.5 Validity and Entailment
Validity and entailment are semantic notions that compare formulas across all structures, rather than inside one fixed structure. Satisfaction tells us whether a formula is true in a particular structure under a particular assignment, while validity and entailment ask whether truth is forced in every possible interpretation of the language. Validity A sentence is valid if it is true in every structure for its language. Definition 2.41 (Validity) Let $A$...