Daniel Finkelstein spells out the illogic of Nick Clegg's position on the euro:

Would he take Britain into the euro? Not now, not soon, but he would
like to keep the door open, is the gist of the answer. Might he want to
join in the lifetime of the next parliament? “I see no circumstances in
which we would do that, no, because it’s not economically justified.”

But . . . “If next Tuesday the world went completely upside down and
inside out and we decided the only way we could secure the prosperity
of the British people is to change our currency arrangements, you have
to keep that option open.”

You can’t have it both ways . . . “You can, of course you can. It’s
perfectly logical and rational to say no; any case for going into the
euro does not exist at the moment. Can I predict at what point it may
or may not be justified in the future? No, of course I can’t.”

The most obvious problem is that he is - within the
space of a few moments - arguing both that he can't see any
circumstances in which he would take the country into the Euro in the
next Parliament and that he can see such circumstances.

But my problem goes beyond that. I don't understand the logic of his position.

He believes that it is not economically justified to take
the country in now. But that it may be urgently necessary at some point
in the future.

Yet once we are in the Euro we cannot come out again. So
the only justification for joining is that, in his view we would never
be in a position in future where being in it it was economically
unjustified.

Yet how can he believe that - that it would never be
economically unjustifiable in future - if he believes that it is
unjustified now?