A man who loved liberty and the law

The life of a barrister who traded wealth and power for philosophy wins Mary Warnock's vote for 'biography of the year'

In the "Biographer's note" that prefaces her Life of H. L. A. Hart , in which she explains her approach and cites her sources, Nicola Lacey says that before she began work she reread Ray Monk's biography of Wittgenstein ( Wittgenstein: The Duty of Genius , 1990). Her book may fairly be compared to his; it shows the same seamless knitting together of the personal with the academic, each illuminating the other, and it is of the same absorbing interest to anyone who cares about the intellectual history of the 20th century. Moreover, she has the great advantage of having known her subject and having been able to talk freely to his widow. Indeed much of the freshness and manifest truthfulness of her writing derives from Jenifer Hart, who emerges almost as clearly as Herbert himself, austere, intelligent, emotional, direct and fierce.

Lacey's account of Hart's childhood is economical, unsentimental but deeply touching. He was the third of four children of a middle-class Jewish family who moved from East London to the ladylike environment of Harrogate, where his father was a furrier and his mother a dressmaker. Her clients would largely have come from Leeds, where it was thought beneath contempt to get your dresses from Leeds itself. Mrs Hart was able to encounter her future daughter-in-law's mother, the exceptionally conventional and snobbish Lady Fischer Williams, without alarm, from experience with her customers.

Hart, always thought exceptional within his family, was sent to boarding school at Cheltenham College, chosen perhaps because it had a Jewish house, where he was unsuccessful and unhappy. But when the family moved to Bradford, he mercifully started at the grammar school where both his elder brothers were pupils, and from that moment he flourished. The account of his success at the school, his love of it and the degree to which his talents and his charm were fully appreciated tells one much about what could be a perfect match between a good grammar school and its pupils.

A scholarship to New College, Oxford followed. Here, most of Hart's friends, and his tutors, were Wykehamists, the connections between Winchester College and New College being much closer then than they are today, though they remain twin foundations. Lacey, perceptively and without making too much of it, emphasises how, throughout his life, and whatever his successes, Hart always felt an outsider. Reading about his undergraduate life, I wonder whether part of this feeling was caused by the domination of the Wykehamists. Though they certainly welcomed him as their equal, their clannishness can sometimes be overwhelming.

Hart came back to Oxford University, with many doubts and reservations, when he was 38, in October 1945. He must have been a completely changed person. He had rejected the possibility of an academic career after graduating and had instead become an extremely successful barrister, making a lot of money and engaging in country life, including hunting, when away from London at weekends. He had been a highly successful intelligence officer during the war, work that he had enjoyed even more than his work at the bar. He was married with two children. Apart from the domestic difficulties centred on Jenifer's continuing job at the Home Office, his doubts were mainly about his own competence as a philosophy tutor at his old college.

He had, after all, taken Greats in 1929, and the philosophy to which he had been drawn as an undergraduate, though the subject papers for which he would have to prepare his pupils remained nominally the same, had changed almost beyond recognition by 1945. He had missed the publication in 1936 of A. J. Ayer's Language, Truth and Logic , and though doubtless he had read it and knew about the shock that it caused among philosophers of the old school, he had missed what must have been seminal discussions for the new style of philosophy, the meetings at All Souls of Isaiah Berlin, Stuart Hampshire, J. L. Austin, Ayer himself and a few others, to discuss the book in detail and pull it apart.

However, he survived, and to all appearances was a huge success. He was certainly a wonderful tutor, and an essential part (whether he recognised this or not) of the close-knit academic and social life characteristic of Oxford at this time. Lacey is skilful at describing the excitement of philosophy in the immediate aftermath of the war, and especially the influence of Austin, of whom Hart became a great admirer and with whom he soon began to give seminars in which he contributed legal examples to illustrate the multiplicity of different ways there were to fend off charges of responsibility when one was accused of crime. These classes were summarised in Austin's paper "A plea for excuses" (1956). But, as he wrote in this paper: "Much of the amusement, and of the instruction, comes in hounding down the minutiae"; and one paper could not give the flavour of these fascinating pursuits. Ronald Dworkin, later to become Hart's heir apparent, attended some of these classes as a conspicuously self-assured graduate student.

In 1952, Hart was elected to the chair of jurisprudence in Oxford. The lessons in clarity, attention to language and to language as illuminating the facts themselves that Hart had learnt as a philosopher (lessons in "linguistic phenomenology", to use Austin's phrase) are perhaps most directly visible in the book co-authored with Tony Honore, Causation in the Law , published in 1959. Since it has always been recognised that it is the ordinary person's idea of cause that is central to assigning causes (and thus responsibility) in the courts, the method of analysing the ordinary use of language and the innumerable distinctions that are drawn within such use was peculiarly appropriate for treating this subject. What became clear was that there is not just one unified meaning of "cause", but a cluster of senses more or less closely related to a central core, that of physical intervention, pushing or pulling, to bring about a certain result. The language of causal connection, especially as used in the courts, is full of metaphors derived from this sense of physical intervention, and part of the aim of the book was to demystify this language and show how it should be understood. A second edition was produced by the authors in 1985, but Hart's major contribution, the first five analytic chapters, remain virtually unchanged. Here he writes predominantly as a professional philosopher, and highly illuminating it is.

In 1961, Hart published The Concept of Law , a far more abstract work, more influential, and more controversial within the history of jurisprudence.

Lacey rightly places The Concept of Law at the centre of Hart's achievement, but also at the centre of the often exhausting and depressing battles he fought to the very end of his life. At the heart of these battles lay the issue of the relation between the law and morality.

Broadly, Hart was and struggled to remain a legal positivist. He held that the law and morality must be separate, though related and overlapping. Like Bentham, Hart believed that the law had its own authority, and created its own obligations, but that it could always be criticised and improved according to separate standards of what was right.

For example, he held, like J. S. Mill, that individual liberty was one of the highest moral values, and therefore the law prohibiting homosexual acts between consenting males should be removed from the statute book. (This view led to a famous battle between Hart and the conservative judge Patrick Devlin after the publication of the Wolfenden Report in 1957.) The law was a social construct consisting of rules, and its authority derived not from the content of the rules but from their source, "a distinctively institutionalised system of social recognition", to use Lacey's words. To be a law, whether enabling or prohibiting, whether conferring rights or laying down valid procedures, a measure must itself have become law by proper and recognised procedures. This was what the "rule of law" amounted to. The aim of his book was a kind of descriptive and analytic sociology, as one might write a book about the concept of education or of religion.

Such a view had no place for the idea of "natural law" nor of pre-existing "human rights".

Increasingly, however, legal positivism came to be rejected, partly on the grounds that, in practice, judges openly employ moral considerations in interpreting law in difficult cases, but largely because of the problem posed by the status of law in evil or corrupt societies. Can the law in such cases be held to have its own unique authority? Dworkin, Hart's successor as professor of jurisprudence, though his views changed fairly radically over time, was a strong defender of the existence of certain inherent human rights that it fell to judges to discover in their creative interpreting judgments.

Lacey's account of the widening gap between Hart and Dworkin, and Hart's manifest unease in confronting his younger colleague, is the thread that links his academic life and his insecure, sometimes neurotic private passions. It is a story that is complicated and sometimes hard to follow; but it has a kind of tragic force. She tells other stories, too, with tact and grace: Hart's deep involvement with his children, including his brain-damaged youngest son; his often prickly but loyal relations with Jenifer; his permanent consciousness of his own Jewish cultural background; and the sad and absurd incident in 1982 of the spying accusations.

All these appear as consistent elements in the portrait. For me, a biography addict, this is certainly the biography of 2004.

Baroness Warnock, a former mistress of Girton College, Cambridge, is a cross-bench member of the House of Lords.