Ever since its formulation by Hume, the idea of an inferential barrier between non-ethical (“is”) propositions and ethical (“ought”) propositions (also known as Hume’s is-ought thesis) has received much philosophical attention. Prior’s Paradox appears to demonstrate that the ban on “is”-“ought” inferences is violated in every possible instance, from which it follows that Hume’s is-ought thesis must be false. In this article, I will formulate a logically rigorous version of Hume’s is-ought thesis, introduce Prior-style counterexamples, and suggest how they might (...) demonstrate the falsity of Hume’s is-ought thesis, identify the substitution criterion, and develop at length the unrestricted and restricted versions of this criterion as it is applied by Pigden and Schurz. Thereafter, I will outline Schurz’s defence of the is-ought gap in accordance with his interpretation of the substitution criterion, before introducing a more sophisticated version of Prior-style objections, proposing a possible response from Pigden, identifying the costs of Pigden’s response and counter proposing a response on behalf of Schurz through an extension of his approach, and offering concluding remarks. (shrink)