This paper seeks to uncover and rationally reconstruct four theoretical prescriptions that H. L. A. Hart urged philosophers to observe and follow when investigating and theorizing about the nature of law. The four prescriptions may appear meager and insignificant when each is seen in isolation, but together as an inter-connected set they have substantial implications. In effect, they constitute a central part of Hart’s campaign to put philosophical investigations about the nature of law onto a path to a genuine research (...) program. The paper takes note of certain prevalent and robust trends in contemporary legal philosophy that detract its practitioners from the four prescriptions, and that have them revert to the some older modes of thinking from which Hart sought a decisive break. A number of contemporary legal philosophers’ views and commitments are taken up and assessed, and in particular those of John Gardner and Leslie Green. (shrink)