Is There Such a Thing as Mass?

In our book, frequently asked questions about the universe, we tackled this in a whole chapter where we basically conclude that there's no such thing as mass. It is all just energy, but it does seem to have inertia. And as to also in other counterintuitive examples, like with photons, photons have no mass. But if you put a bunch of photons in a box with mirrors inside, for example, so they're bouncing around, then that will have more mass than an empty box.