RG Collingwood painted by his mother, 1912

Eighty years ago, in the summer of 1939, Oxford University Press published An Autobiography by R.G. Collingwood. It was a modest-looking book, not much more than 150 pages, and no one expected it to do well. The fact that Collingwood held the Professorship of Metaphysical Philosophy at Oxford was not going to attract many readers, and in any case he was a marginal figure at his university, not to mention the rest of the world.

Collingwood gave due warning to his publishers: he was essentially a thinker, he had not lived the kind of life that "makes autobiography saleable", and they should brace themselves for a "dead loss".

In the event, however, An Autobiography sold like hot cakes: it was not only deliciously readable, but also intellectually substantial and politically explosive: in short, one of the most powerful philosophical works of the century.

The opening pages evoke a bohemian childhood in the Lake District, with artistic, bookish parents who treated their children as their equals. Collingwood took part in their music-making, commented on their paintings, and taught himself natural history by wandering round the countryside. He picked up Latin and Greek from his father, and helped him with some important archaeological digs. He had his first philosophical experience around the age of eight, when he came across a tattered 17th-century book about the natural world, and realised with a shock that what seems self-evidently true at one time may seem self-evidently false at another. The memory of his philosophical awakening never deserted him.

He went to school at the age of 13, but found it intellectually suffocating, and he was relieved when he got to Oxford and reclaimed his creative autonomy, first as a student and then as a tutor and professor. But he never thought much of his philosophical colleagues: most of them were dull and obtuse, which was bad enough, and the rest were pert and argumentative, which was even worse. "I have always been a slow and painful thinker," he said, "in whom thought in its formative stages will not be hurried by effort … but grows obscurely through a long and oppressive period of gestation." Premature argumentation, he found, was a "dangerous enemy" to intellectual originality.

As years went by, the idea that philosophy must keep close to history remained the touchstone of all his intellectual efforts. His colleagues were bemused by the fact that he remained an active archaeologist, and managed to write authoritative books on Roman Britain in his spare time. They were indifferent when he began to expound what would become his signature doctrine: that intellectual change comes about not when we find new answers to enduring questions, but when our questions grow old and give way to new ones. And they did not like it at all when, after visiting the Spanish republic in the early 1930s, he decided he wanted to be a "fighting philosopher", rather than a "professional thinker, safe behind the college gates". Anyone who took the historicity of thought seriously, he said, was bound to become a militant anti-fascist and an uncompromising defender of "liberalism" and "democracy".

Collingwood’s preference for democracy over fascism would have been shared by many of his philosophical colleagues, if not a majority, but none of them approved of the way he tried to connect philosophy with politics. As philosophers, they thought of themselves as intellectual royalty, with exclusive access to the highest principles of human conduct, and a noble obligation to visit the vulgar world of politics from time to time, in order to instruct ordinary people in the philosophically correct way to behave.

Collingwood did not share these delusions of grandeur. He did not believe that philosophy could put people in contact with eternal principles of politics or anything else, and it was for precisely that reason that he deplored fascism and advocated liberalism and democracy.

He did not see democracy in the traditional way, as one form of government amongst others. As far as he was concerned, "the democratic system was not only a form of government but a school of political experience" – a school which generates a "politically educated public opinion" which will in its turn fight to defend democracy. We cannot know in advance what kinds of policy will suit us, but democracy gives us the opportunity to imagine new ones and try them out. Democracy, as he saw it, is like "a nursery-garden where policies are brought to maturity in the open air, not a post-office distributing ready-made policies to a passively receptive country".

But the democratic system was fragile, and it would be destroyed if the electorate allowed itself to become "ill-informed on public questions", thus ceding power to "some irresponsible cabal". And that, he thought, was exactly what had been happening in Britain since the Great War. The electorate had been corrupted by political clowns like Lloyd George, Ramsay MacDonald and Stanley Baldwin, who entertained the public with evidence-free performances in which "nothing was definitely said, but a great deal was hinted".

The real danger however came not from the politicians but from the press, especially the Daily Mail which since the 1920s has devoted itself to presenting political news not as a collection of "facts which a reader ought to know if he was to vote intelligently", but as "a mere spectacle for idle moments". The Daily Mail, according to Collingwood, was a massive machine for "corrupting the public mind".

Collingwood wrote those words with a tremendous sense of urgency. "The country has been tricked," he said. The British government, enslaved by the Daily Mail, had already waved through a Fascist coup in Spain, and was now busy appeasing Hitler.

But there was another reason for urgency. Collingwood was not yet 50, but he had just suffered a stroke and knew he did not have long to live. An Autobiography was designed as his dying message to the world. He struggled on for longer than expected, but his death in 1943 at the age of 53 deprived the world of a calm democratic intelligence that we could have done with in the following decades, and which might be even more salutary today.