Massachusetts Institute of Technology: Srini Devadas and Eric Lehman's "Logic"
Read Section 1.2 on pages 5 and 6 and 1.4 on pages 6 - 8 to learn about logical equivalence of statements. The term validity refers to an argument or proof. We say an argument is valid if its conclusion logically follows from its premises. We can do this by showing that a statement is logically equivalent to a premise, or by showing that a statement logically follows from a premise. Logically follows from, or is a consequence of, means that the conclusion is true if the premise is true.