It's the sheer randomness of it that makes it so terrifying. The idea that the victim would be utterly generic: not a politician or a soldier, but a random person. This person would be an abstraction, really. They are everyone precisely because they are no one in particular. What would matter is the image. The video had to be made, the event had to be broadcast. This matters more than the killing itself.

Now will come the courtroom arguments about whether this is a serious plot or empty, youthful bravado, but they will have no public purchase. Terrorism is all about the fear, the anxiety, the outrage. It's nothing without it. And what can scare or outrage us more than the thought of ISIL within?

"ISIL is not simply a group to be vanquished ... It exists in the mind as much as on land." Credit:Reuters/Australian Federal Police

And it's that thought that perhaps has the most to teach us in Australia. ISIL is not simply a group to be vanquished. It is not a fixed, finite, collection of people we can somehow control or eradicate. For us in Australia, it's most dangerously a symbol: a brand a young man from Sydney can claim for himself; a flag in which he can wrap himself, and his proposed victim. For all its pretensions to statehood, the key thing is that it's anything but. It exists in the mind as much as on land.