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Ronald Dworkin is HLA Hart's successor to the Chair of Jurisprudence at Oxford, and one of his most determined critics. According to Dworkin, the whole of Legal positivism rests on an unsteady foundation - the notion that there is a rule that can unambiguously classify rules into 'law' and 'non-law'. Hart called this foundational principle the 'rule of recognition'; Kelsen described a related concept, which he called the grundnorm. Although the 'rule of recognition' and the grundnorm are conceptually different, both the serve the positivistic end of distinguishing laws from non-laws.

Dworkin asserted that judicial practice simply did not support the notion that law was a body of rules. He described decision-making instead as a tension between rules and 'principles'. In any given case, rules might conflict with principles, and principles with each other. Then the judge would have to decide the relative weight to apply to each. For example, in the case of Riggs v Palmer (1889) the court had to decide whether a man who had murdered his grandfather could inherit under the grandfather's will. It was clear that the standard rules of inheritance said that he could, indeed there was no rule of law that said he could not. However, it was a principle of law that courts should not allow bad guys to profit from their own wrongdoing. This principle cannot, according to Dworkin, be reduced to a rule. Clearly the law does not impose a rule that bad guys cannot profit from their wrongdoing - he cites the law on Adverse possession as an example. Similar, in general a principle cannot be expanded to a body of rules, as the category of affected decision is not closed. So, according to Dworkin, positivism's central tenet is unfounded.