From 1966 to 1983, Peter Laslett, who has died aged 85, was reader in politics and the history of social structure at Cambridge University. He was also, with Michael Young, one of the instigators of the Open University in the 1960s, and of the University of the Third Age in the 1970s. In 1964 he was co-founder - and director - of the Cambridge Group for the History of Population and Social Structure. Laslett acquired a worldwide reputation, but his university never awarded him a professorship. He went his own way, far more gifted than many who attained professorial rank, an original thinker, and a trenchant, elegant writer.
A clergyman's son, he was educated at Watford Grammar School and St John's College, Cambridge. From 1940 to 1945 he was in naval intelligence. He then became a BBC Third Programme producer.
In 1948 he was elected to a fellowship at St John's and began pathbreaking research on the social and political theories associated with the constitutional upheavals of 17th-century England. Laslett recognised that critical commentary on the philosophies of this climacteric period - especially on the philosophies of Thomas Hobbes and John Locke - rested on absurdly shaky foundations. There were no reliable modern editions of the relevant texts.
Laslett began rectifying this in 1949 with his edition of Sir Robert Filmer's political writings. In 1960 came his definitive, critical edition of Locke's Two Treatises of Government (1960), which established the standard for the editing of such works.
During the 1950s Laslett was no less interested in contemporary political philosophy than in the history of political thought. He lectured at Cambridge with panache and originality - when he remembered to turn up - and in 1956 initiated Philosophy, Politics and Society, a series of collections of essays that has flourished under his general editorship ever since.
His early studies in intellectual history continue to be widely influential. Perhaps his most significant achievement was to place the theories of Filmer, Locke and their contemporaries within their political contexts. This approach was brilliantly developed by a number of Laslett's students, and still flourishes at Cambridge.
In the 1960s Laslett's research took a different turn. Social structures, he decided, mattered far more than individual thinkers. Statistical analysis, not textual interpretation, was the future. It was an exciting time for those interested in the history of population and social structure. Fundamental to clearer insights into the differences between the pre- and post-industrial worlds was Laslett's questioning of assumptions about the nature of the family and household in early modern western Europe.
Laslett wished to discover how far English social and familial practice reflected Filmer's prescriptive patriarchalism. What he discovered shattered many beliefs about pre-industrial society. Families were predominantly nuclear, a married couple and their children. Households were small. Three-generation households, and households in which kin such as uncles, cousins or married siblings were present, were rare. Only one non-nuclear element was commonly found in early modern English households: living-in servants. These were not servants in the Victorian domestic sense, but chiefly servants in husbandry, young people learning skills and acquiring resources which might in time enable them to marry. Children left their parents' household in their mid-teens and spent much of the next decade of their lives in service. Servants were unmarried and both sexes married in their middle or later 20s. The "Juliet" syndrome, confined to the elite, was largely disappearing even there before the end of the 17th century.
The widely held view that the small, nuclear family was a product of industrialisation and urbanisation became untenable in the light of Laslett's work. He also showed conclusively that early modern English society was highly mobile: only a relatively small minority of each rising generation lived lifelong in the same parish.
His commitment to broad issues of historical interpretation did not preclude taking an interest both in methodology and in an immensely important area of technical advance, using modern computing to simulate the behaviour of populations, to study both demographic and social structural issues, and the interaction between the two.
In the later 1980s Laslett became interested in aspects of the ageing process. He explored the distinction between the "third" and "fourth" ages and argued vigorously against the tendency to push those above working age to the periphery. A Fresh Map of Life (1989) provided a new vision of a topic encrusted with platitudes.
Laslett had a remarkable gift for commanding an audience beyond academia - a reflection perhaps of his Third Programme days. A veteran Workers' Educational Association teacher observed that no other book recommended to his classes had ever been so welcomed and praised as The World We Have Lost (1965), which reported what Laslett had learned about English social structure, foreshadowed much that was to come during the next 25 years, and provided, in an accessible form, the concepts that gave meaning to the facts he described.
Laslett never lost his curiosity and passionate commitment to the life of the mind. Nor did he ever seem to age. This year he took part in a Cambridge symposium (shortly to be published) on the current state of political philosophy, astonishing everyone with his energies and his quirky and challenging judgments.
He married Janet Crockett Clark in 1947. She survives him, as do their two sons.
Thomas Peter Ruffell Laslett, academic, born December 18 1915; died November 8 2001
Michael Young writes: The story in his family is that, at the start of his lectures, Peter Laslett would take off his coat, take off his gown, take off his jacket, unbutton his shirt sleeves and roll them up, while the audience sat frozen in anticipation that, this time, he would take off all his clothes. Perhaps it was his wartime Japanese naval intelligence codebreaking at Bletchley Park that gave him a taste for action.
In the late 1950s Peter, as much in search of equity and the expansion of higher education as me, published a proposal in the the BBC's Listener magazine for a university of Great Britain. He wanted to divorce the academic success of Oxford and Cambridge from their social esteem by making them postgraduate institutions. The staff of all other universities would be considered as staff of Oxbridge as well.
That did not get far. But from my own arrival in Cambridge in 1958, as a lecturer in sociology and fellow of Churchill College, I fell into league with him as a reformer. He and the classicist John Morrison were the only Cambridge supporters of my proposal to found a second university on the same site, which would be in session when members of the existent university were on vacation. "Vacations?" came the response."That's when we do our real work, when our students have gone away."
I then decided, with Peter's agreement, that an open university should not be grafted on to an existing university. With Brian Jackson, we eventually started the National Extension College, which now flourishes long after it fulfilled its first role as a pilot project for the Open University. Peter remained trustee of the NEC until 1990.
In Humphrey Carpenter's The Envy of the World: Fifty Years of the BBC Third Programme and Radio 3, Peter explained his espousal of the OU: "Michael Young and I became friends. He and I agreed about breaking the monopolies of universities, and broadcasting was clearly the manner of doing so. A submission, signed by distinguished names, persuaded Harold Wilson that the University of the Air was not only practicable but an obvious Labour party policy. We knew even then that it had to be a mixture of television, open-circuit sound broadcasting, closed-circuit recording, correspondence and teaching. We wanted broadcasting because it would be an immediately, completely transparently available source of instruction to every citizen with a wireless or television set."
For the new project, Peter visited the United States in 1961, on a Dartington Hall grant, to look at educational television. He was the leading organiser of a 1963 week of early-morning television launched by Fred Hoyle, Plumian professor of astronomy at Cambridge, and seen by 200,000 people. It almost certainly influenced the OU's later use of broadcasting. He also led a project on sharing teaching and research between universities through the new technologies.
Peter was a member of one of the committees advising Harold Wilson's arts minister Jennie Lee on the OU's shape. The story is that he told Jennie that he assumed the committee members came with an open mind. He would not have been invited, she replied, if she had known he would ask such a question.
In 1982, we joined up again, as founders - with Eric Midwinter - of the University of the Third Age, the only more or less self-financing university almost anywhere for people over 50 who largely teach each other. Faculty keeps changing into students. It has 472 branches and more than 112,000 members. Peter was that too-rare bird, an academic of high standing and also an entrepreneur in that world, who was intensely interested in the people who do not get to their universities. If only there were more like him.