Even if we could establish a universal objective good (see our Ergo), we cannot logically claim, or convincingly argue, that we must "do" good.

In his 1912 essay "Does Moral Philosophy Rest on a Mistake?", Prichard said we cannot proceed from non-normative premises to a normative conclusion. Moral obligations are natural common sense attitudes that are self evident and that we self-impose. This is Hume's "naturalism."

Prichard argues that no rational or logical proof is available to justify a moral obligation.

Moreover, he says the search for justification in ethics is as futile as the search for ultimate justification in epistemology.

If we reflect on our own mental history or on the history of the subject, we feel no doubt about the nature of the demand which originates the subject. Any one who, stimulated by education, has come to feel the force of the various obligations in life, at some time or other comes to feel the irksomeness of carrying them out, and to recognise the sacrifice of interest involved; and, if thoughtful, he inevitably puts to himself the question: "Is there really a reason why I should act in the ways in which hitherto I have thought I ought to act? May I not have been all the time under an illusion in so thinking? Should not I really be justified in simply trying to have a good time?" Yet, like Glaucon, feeling that somehow he ought after all to act in these ways, he asks for a proof that this feeling is justified. In other words, he asks "Why should I do these things?" and his and other people's moral philosophising is an attempt to supply the answer, i.e. to supply by a process of reflexion a proof of the truth of what and they have prior to reflexion believed immediately or without proof.

This frame of mind seems to present a close parallel the frame of mind which originates the Theory of Knowledge. Just as the recognition that the doing of our duty often vitally interferes with the satisfaction of our inclinations leads us to wonder whether we really ought to do what we usually call our duty, so the recognition that we and others are liable to mistakes in knowledge generally leads us, as it did Descartes, to wonder whether hitherto we may not have been always mistaken. And just as we try to find a proof, based on the general consideration of action and of human life, that we ought to act in the ways usually called moral, so we, like Descartes, propose by a process of reflexion on our thinking to find a test of knowledge, i.e. a principle by applying which we can show that a certain condition of mind was really knowledge, a condition which ex hypothesi existed independently of the process of reflexion.