An argument or inference is an entailment iff. Its conclusion conclusively follows from its premises. A set of premises entails a conclusion iff. The conclusion conclusively follows from the premises. If p then q = only if q then p. Only if p then q = if q then p. If and only if - is a biconditional, combination of both if and only if. Argument may be valid, may be invalid; insufficient information. Inference from the premises to the conclusion - good. Therefore, the moon is made of green cheese f. Inference from the premises to the conclusion - bad. Therefore, all frogs are good at logic f. If today is wednesday then tomorrow is thursday. *all arguments t|f, t|t, f|t, f|f can be invalid; its invalidity depends on its inference from premises. *all arguments can be valid except for t|f; its validity for the other three cases depends on its inference from premises.