Earth-shattering thinkers (Archimedes, Newton) had a single and major eureka moment, in the bath or under an apple tree. Jobbing scholars (like me) are more likely to have numerous minor insights in different places (in my case, in the shower, perhaps, or in the witching hours of the night). None of my eureka moments has changed the world. They are not even on a par with the inspired discovery that putting clingfilm in the fridge makes it much easier to use.

My first brainwave was doubtless common to every postgraduate struggling to win a research scholarship or temporary lectureship. It was that your ability counted less than the political power of your referees.

My second came in 1971 when I was elected Michael Foster scholar at the University of Oxford and was interviewing people for a thesis on the subversion of the Weimar Republic by Nazis and Communists. I suddenly realised that many people of influence who had played a part in this story and had agreed to speak to me were, on the inside, not what they seemed from the outside.

Indeed, more frequently than one may suppose, many were linked to British Intelligence, both during the Second World War and subsequently. This was true not only of several Oxford dons (for example, James Joll and Stuart Hampshire), but also of some of the Germans, such as Fritz Heine and Stefan Thomas. Both had been wartime members of the Special Operations Executive and retained secret contacts with our scholarly community. One Oxford academic, Christopher Hill, had worked for the SOE and for Soviet intelligence (I kept his confession secret as long as he was alive). You would not have guessed any of this at the time.

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The recent media interest in my work has often come from the eureka moment I had (when shaving) in 2002, pondering the emerging problem (as it then was) of Islamist terrorism. I had just completed the manuscript of my book on the Stasi (The Stasi Files: East Germany's Secret Operations Against Britain), describing, inter alia, how in the UK the security service had exploited academic space, British students and academics - people interested in ideas - to further its particular aims.

The Stasi did not support terrorism in the UK (although it did so in West Germany). But in our universities, it found a source of both hard agents and "useful idiots" - academics hungry for research "findings" supplied by the East Germans and useful for research assessment exercise purposes, who were only too happy in return to peddle the nonsense that East Germany was the "better" Germany, outstripping Western democracies in cultural and economic terms. They were the willing helpers of a totalitarian secret police and an odious political system - and they were well educated.

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The events of 11 September 2001 represented an obvious and serious intelligence failure. But it suddenly occurred to me that we needed to examine the background of the 9/11 bombers and not just regard them as appalling criminals, understandable as that view might be.

After all, several of the 9/11 operatives had been students (notably at Hamburg University) and al-Qaeda used graduates more generally, as well as "useful idiots". If Islamic terrorists were not just criminals but also students or graduates, campuses and colleges could be places to look for them. Of course, many universities contain both the bright and the idiotic, dons as well as students.

The next eureka moment came in 2004. If Islamic terrorists, even just a few, were stalking our campuses, we needed to find a means of preventing them from radicalising others. If educated people were turning to terror, it must follow that their education was lacking in some major way.

Universities ought therefore to be doing far more to convince their students that change in our country must be peaceful and democratic. If dons were too cowardly to do so themselves, the government should instruct them to act (after all, the taxpayer was funding them).

Rather than turning a blind eye to what was happening on their campuses, and focusing on research that few, if any, would read (unthinkingly following the mirage that is research funding), academics needed to get back to teaching their students - and not just their special subjects, but about life and liberty as well.

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I argued this at the Political Studies Association annual conference at the University of Lincoln in 2004. When I returned from the conference, the vice-chancellor of my university at the time, Brunel, rang me at breakfast to say that he'd read a report of my talk in the Financial Times and that I might like to know that some Brunel students had been arrested on terrorism charges earlier that week. They were later to be known as the "Crevice" cell.

He was kind enough to congratulate me on my predictive skills (knowing I was completely ignorant of this plot).

My next (and related) eureka moment took place in 2006: if Islamic terrorists came from two discrete groups, and one group was homegrown, then might they be trying to construct within campuses sites that would support radicalisation, and then bring thugs into the UK, perhaps posing as students, who were ready to bomb when the order came through?

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This thought led naturally to the next thought: if there were a "campus" strand (and I never suggested it was more than a strand), was it being sustained by external funding? Together with my colleague Julian Richards (at that time research fellow in Brunel University's Centre for Intelligence and Security Studies), I looked into the funding issue and noticed that some ?250 million of Arab and Islamic funding had poured into UK universities since 2000, most from Saudi Arabia, most to the University of Oxford, and most to support Islamic centres. Even philanthropic projects had an Islamic propagandistic side, as the rebuilding of the Ashmolean Museum in Oxford readily illustrates.

My final eureka moment? It came just a few days ago. I'd been reading Alone in Berlin by Hans Fallada, in a brilliant translation by Michael Hoffmann. Based on the true story of the hunting down by the Gestapo of an anti-Nazi in wartime Berlin, it is required reading for anyone interested in the essential difference between intelligence-led activity tasked to underpin a vile totalitarian regime, and intelligence-led activity designed to uphold democracy.

What I suddenly saw was that in fighting those who are democracy's enemies (Islamists, Nazis, animal liberationists), we need an MI5 to protect us fully, but that the police should not be doing the intelligence service's job for it. Policing should be kept distinct from spying. It's vital, too, that unlike our American friends, we eschew utterly the use of torture, since to torture as a means of social control (as the Gestapo did) is a mark of totalitarianism, not democracy.

But what really makes MI5 and Special Branch different from the Gestapo is the political purpose that drives what they do.

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