H.L.A.Hart, a philosopher of law, first developed this example in a lecture published as The Separation of Law and Morals, 71 Harv. L. Rev. 593, 607 (1958), and developed in his book, The Concept of Law 121-32 (1961). Lon Fuller, a professor at Harvard Law School, disagreed with Hart's notion that the rule proscribing vehicles in the part had a clear core meaning and instead suggested that the border between core and periphery could be easily obscured. See Lon Fuller, Positivism and Fidelity to Law -- A Reply to Professor Hart, 71 Harv. L. Rev. 630 (1958).