But the root of the problem was not just human nature.

But he really can't get at the root of the problem.

I'm not sure what was at the root of the problem.

Let us go back to the root of the problem.

He wanted to get to the root of his problem.

This does nothing to deal with the root of the problem.

Still, numbers were thought to be at the root of the problem five years ago.

And that seems to be at the root of the problem.

This would not, however, get to the root of the current problem.

One can spend a long time thinking about where the roots of the problem are.