# How do they know an integral has no closed form solution? [duplicate]

Possible Duplicate:
How can you prove that a function has no closed form integral?

When they say that, e.g., Li(x) has no closed form (for some agreed upon definition of "closed form"), do they mean that it can be proved that there isn't one, or just that none has been found yet. Most authors seem to just assert that a given integral can't be represented in any simpler way, but I've been wondering if that's because they know this for a fact, or just that it's assumed that if there is one it would have been found by now.

If this is a theorem, how would you go about proving it?