In this paper I advance a completely general account of what it means for a conclusion to follow from given premises: An acceptable counterfactual-supporting covering generalization of the argument rules out, either definitively or with some modal qualification, simultaneous acceptability of the premisses and non-acceptability of the conclusion, even though it does not rule out acceptability of the premisses and does not require acceptability of the conclusion independently of the premisses. Hence a supposed unexpressed premiss supplied to make an argument formally valid should be a covering generalization.