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The prime minister is walking a tricky path. He has to convince Brussels that he is serious about campaigning to leave if he doesn't get his reforms. Otherwise he has no leverage.

But he also has to convince them that he will campaign to stay in if he gets the changes he wants. And sometimes that means arguments get confused.

For example, today he argued that the EU plays a strong role in national security. If so, how could he contemplate leaving?

There are also contradictions. Much of the renegotiation will be about finding ways of curbing migration within EU rules. But Downing Street is keen to focus its subsequent referendum campaign on other issues, such as protecting people's jobs.

The truth is that David Cameron will get some kind of reform.

It won't be enough to satisfy his critics. The question that matters is whether it satisfies enough people in a referendum to vote to stay in.

And on that hangs not only Mr Cameron's future as prime minister but also Britain's future relations with the EU and the rest of the world.