Garvan Walshe is a former national and international security policy adviser to the Conservative Party. He runs Brexit Analytics.

Why does Jeremy Corbyn persist in blaming British foreign policy for terrorism? The answer is to be found in Marxism – not just his own Marxism, but in Marxist influence over the social sciences, which, in their failed attempts to mimic the precision of the physical sciences, persist in ignoring the human factor in political violence.

There’s a disingenuousness at the heart of Marxist doctrine. Marxism. Its rhetorical power came from denying its own power as a rhetorical device. Marxism didn’t persuade, it described.

Though Marxism failed as a social science, its predictions proven wrong by experience, it lives on as a vehicle for ideas about economic organisation, and exploitation and a formulation of the oldest populist cry: they, the elite, have rigged the system against you.

More than that, it lives on in debased form in the vernacular of social science. People, Marx himself said, are free to make choices, but not in circumstances of their own choosing.

This may be a useful corrective to the ideal that we are unencumbered by the facts of life. None of us can control what people did before we were born: we’re stuck with the consequences of their decisions. We can’t escape the actions of the rest of society: they obviously affect us. Libertarians, of course, oppose this fact and want to reduce its ability to affect our individual freedom, but they’d be mad to deny such encroachments on liberty exist.

Yet this reality is too easily overlooked in our discussions of terrorism. Last week, Corbyn gave a speech linking the Manchester bomb and British foreign policy (there is a real link – British foreign policy helped remove the dictator that exiled the bomber’s family; when the Didsbury mosque’s imam was videoed in combat fatigues exhorting the faithful to fight Gaddafi, he was in alliance with British foreign policy, rather fighting than against it; not since Donald Trump fired James Comey has anyone been so ungrateful). Corbyn presented what’s become a conventional wisdom – that terrorism has objective causes, capable of investigation by generalised scientific methods: terrorists are supposed to share certain social characteristics or psychological profiles; they respond to external events like lab animals to stimuli. They are “forced” by our actions to undertake such extreme measures.

This failure to ascribe moral responsibility to terrorists too easily slips into justifying terrorism, by placing responsibility on the only people deemed to possess it – the Western powers. Equally, it can be used to justify foolish anti-terrorist policies, by treating it as an inexplicable and irrational force to which no reasoned response is possible.

Both errors look at the circumstances – the terrorist’s background, psychological make up or political environment shaped by their enemy. They give false hope that terrorism can be eliminated by better foreign policy or education or some other administrative policy mechanism (as we might eliminate disease through better sanitation).

The truth, however, is that terrorism is political activity. Of course terrorists act in circumstances not of their own choosing. But they also act – they choose to commit their crimes (or incite or indoctrinate) because they believe they are justified.

They kill for ideological reasons. Ideologies can’t be eliminated by the right antiseptic policy: they have to be argued against and refuted, as well.

Even if they sometimes refute themselves – ISIS, like the Khmer Rouge, has become a self-discrediting movement – we can hasten their defeat by competing with our own narrative. We need to defend our way of life as better than the one the terrorists try to convert people to. This has to go beyond the mere assertion of British (or indeed Western) values. The point about the values of liberal democracy is that they are defensible on their merits, and available to anyone, from whatever background. We’ve chosen them to be ours because they are good: they’re not good just because they are ours.

This, Corbyn doesn’t understand. The ultimate irony, perhaps, is that he is an ideologue himself, and his own ideology blinds him to fact that terrorists have their own ideological motivations. Their struggle isn’t a response to us, but an attempt to impose their destructive way of life. It’s a struggle we need to oppose and defeat, like we defeated the Communist tyranny that once could command millions of adherents to its evil, and in front of whose banner Corbyn was literally happy to stand under just last year.

We had better hope that YouGov’s polls are wrong, and the British people do not catapult someone so comfortable with totalitarianism to Number 10.