The hypothesis of this paper is that legal positivism depends on the non plausibility of strong moral cognitivism because of the non necessary connection thesis between law and morality that legal positivism is supposed to acknowledge. The paper concludes that only when based on strong moral cognitivism is it consistent to sustain the typical non-positivistic thesis of the necessary connection between law and morality. Habermas’s Philosophy of law is confronted with both positions.