According to the Slate review of a new book, Quantum:
Much of the debate between Einstein and Bohr revolved around Einstein's
intuitive rejection of the implication of the Copenhagen interpretation -- which
is that objective reality, independent of any observer, doesn't really exist.
Bohr, by contrast (and sounding a lot like Wittgenstein), insisted that physics
isn't concerned with what is but solely with what we can say about it.
If a tree falls in the forest and nobody's there to hear it, does it make a sound?