Of the main formative influences on my present work as a lawyer, the first in order of time was my experience as a professional teacher of law. As many people have discovered, there is no better way of learning than teaching. At Oxford in the early 1950s I was expected to be able to teach all subjects in the syllabus for Law Moderations, the Honour School of Jurisprudence and B.C.L., unless I could field an adequate substitute. I believe that, with 48 law students as my sole responsibility, I taught about 10 subjects in my first term. One or two subjects I studied for the first time with my pupils, confessing my inadequacy as I did so: there are, I may say, worse ways of teaching than that because, paradoxically, just as teaching students is an excellent way to learn so learning with students is an excellent way to teach. At all events, in these rather strenuous circumstances (which I believe no longer exist), my experience and my knowledge grew apace; and, as they grew, one thing in particular struck me—that, to understand a legal subject, it is necessary to penetrate it in depth at a certain point or points. A skim of superficial knowledge is not enough; and knowledge of the whole subject in depth is neither necessary nor, indeed, possible. Moreover, it is usually not difficult, in my experience, to identify the points of entry. In the law of torts, for example, there would probably be a consensuus in favour of negligence—perhaps with one or two attendant, complementary, satellites. But in the law of contract my choice, perhaps more idiosyncratic, would be twofold: first, that loose bundle of topics usually grouped together under the misleading title of mistake, and secondly, frustration.

Why, it may be asked, choose those two? My answer is that the study of each illuminates so much. Let us take, first, mistake. In my time as a student, the theory of Professor Cheshire held sway over the minds of my mentors. With its three categories of common, mutual and unilateral mistake, the theory was neat, symmetrical and compelling: and when we read the chapter in the First Edition of Cheshire and Fifoot, expounded so clearly in Professor Cheshire’s limpid prose, enlivened occasionally by shafts of Mr Fifoot’s waspish humour, we were easily persuaded, in our innocence, that here was the true light, unperceived by Professor Cheshire’s stumbling predecessors. Then came the decision of Mr Justice Tucker in Sowler v. Potter 1. The decision and the theory were not easy to reconcile: and the reaction of our teachers, and indeed I fear of Cheshire and Fifoot themselves in their next edition, was not to reexamine or qualify the theory, but to reject the decision—a type of error of which we have all, perhaps, been guilty from time to time, when we are too much enamoured of an idea to remember the prosaic truth that judges are paid to react as professional men to fact-situations, and not to invent their own or to regurgitate other people’s theories. Next, the wreckers moved in, in the persons of Sir Owen Dixon and Sir Wilfred