Legal positivism is an influential descriptive theory of law; its main claim, in brief, is that law and morality are separate. Along comes David Gray Carlson, a law professor at Yeshiva University, who recently published this thought-provoking paper on “Legal Positivism and Russell’s Paradox.” His paper on legal positivism is insightful, original, and non-trivial because it applies ideas from the world of mathematics–Russell’s Paradox and the axioms of Zermelo-Fränkel set theory–to the theory of legal positivism. But it was the following sentences in particular that stood out the most in my reading of his wonderful paper:
“What is positivism? In recent decades it has been difficult to say. But perhaps the core belief … is that law is not necessarily connected with morality. What is morality? Scandalously, positivim fails to provide an adequate definition beyond the claim that law is not necessarily connected with it.” (*)
Did Professor Carlson just stab the vampire-like theory of legal positivism in the heart with a stroke of his pen? (I say “vampire-like” since positivism attempts to suck morality out of the law.) At the very least Carlson has shown that legal positivism is a feeble theory of law … because without a working definition of ethics or morality, there is no way to test whether positivism is true as a descriptive matter!
(*) D.G. Carlson, “Legal Positivism and Russell’s Paradox,” Washington University Jurisprudence Review, 5(2): 257-288 (2013), p. 260 (footnote omitted). Cartoon courtesy of xkcd
prior probability 
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