Abstract. I ask whether the validity of a Modus Ponens inference is any part of the explanation of why the inference is, in any sense, a good or valuable inference. I argue for the following. On the orthodox understanding of validity, a semantic understanding, validity has no explanatory relevance to the reasoning’s value. On a metaphysical understanding of validity, one definable in terms of possible worlds, validity is relevant to a partial, but crucially incomplete, explanation of the reasoning’s value. The complete explanation of the reasoning’s value must also appeal to a substitutional understanding of validity, a notion once advocated by Quine.