One of the merits of the internet is that it gives us, for the first time, immediate access to the public utterances of our politicians. So when you see a media storm over something like Malcolm Turnbull’s 11 July London speech, you can just click through and read it. This is the sort of instant gratification that makes today, in Turnbull’s words, an exciting time to be alive.

This speech is worth it, too. It has become weirdly controversial for its alleged downplaying of Liberal Party conservatism. Yet it seems the sort of smart, useful speech we should have more of, exploring basic principles behind policies. Even if you end up disagreeing with big chunks of it, you’ll understand today’s political arguments better.

And it is most interesting in seeking to draw to Turnbull’s cause the ideas of the late Karl Popper.

Popper was born in 1902 and died in 1994. His advocacy of openness and democracy – along with his thinking about knowledge itself – made him in some ways the defining mind of the past 100 years. He is better known as a social liberal than as a conservative, but no-one has bettered his philosophical dismantling of Marxist thought.

As Turnbull and his speechwriters have realised, Popper has useful things to say to any society struggling to work out how far it should go in fighting intolerance. This is just the problem terrorism forces us to confront.

In life, Popper experienced the destruction of a great tolerant society up close. The Vienna of Popper’s boyhood was the world’s glittering centre of innovation in art, science and ideas of all sorts – an intense premonition of the vibrant, networked intellectual world we have today. Yet by the 1930s, communism and the backlash of fascism were tearing it apart; by 1945 many of its best were fled or dead.

The Open Society and It's Enemies

In his most famous book, The Open Society and its Enemies – written after fleeing Austria for New Zealand – Popper clearly felt the need to confront this as a real-life problem. Many Austrians, to their shame, had embraced Hitler, fascism, and mass murder. How should such an open society have properly reacted to citizens clamouring for tyranny?

As Turnbull points out, Popper’s answer was that democracies must develop institutions that reject and prohibit intolerance – “the paradox of freedom”.

Turnbull’s purpose, of course, is to use Popper’s ideas in defence at least of the idea that anti-terrorism policies can be tough without being illiberal. And indeed Popper seems well-suited to this aim. In a footnote to his discussion of the paradox of freedom – and his footnotes are some of his best work – Popper goes furthest of all:

Unlimited tolerance must lead to the disappearance of tolerance ... If we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them ... We should claim that any movement preaching intolerance places itself outside the law, and we should consider incitement to intolerance and persecution as criminal.

And yet ...

Reading Popper, it is also clear that his fear is of intolerant mass movements that threaten to overwhelm democracy. The citizens of 1930s Vienna gave in to Nazism in horrifying numbers. The best analogue for today’s terrorist threat is not fascism but the global outbreak of anarchist terrorism in the late-1800s and early-1900s, not Popper’s intellectual territory.

And for all his warnings, Popper is wary of prohibiting speech about intolerant philosophies: so long as public opinion keeps them in check, he argues, “suppression would be most unwise”.

Malcolm Turnbull is probably right to call terrorism “the most pressing security challenge of our time”. And Karl Popper’s writing reminds us of the need to defend a tolerant society.

But Popper’s life also reminds us of how far terrorism is from posing an existential threat to developed nations. Today’s extremists are far outnumbered by a tolerant majority. While that continues, we should beware overreaction.