A participant in Yahoo!Answers recently asked this:
In philosophy, how does an atheist idealist react to the criticisms faced, that Berkeley can only reply to using God?
It is a provocative question, and even knowing that I was probably being too helpful regarding someone's homework assignment I couldn't refrain from a reply:
I'll try to reconstruct what you mean here, then provide a three-point answer,
Berkeley said material substance does not exist, matter is only the perception thereof in some mind.
This raises the famous question about the tree in the Quad. Does it exist when no one is around?
Berkeley says that it does still exist, and for this purpose he brings in the always-perceiving Mind of God.
So what you mean to ask is: can an atheist coherently believe that Berkeley was right about the non-existence of material substance, and so the non-existence of unperceived matter, and what would THAT person have to say about the tree in the Quad?
The first point to make in answer to this question is that "atheism" is defined only by negation. An atheist denies what some particular theist asserts. Otherwise, even if we're sure they're denying the same thing, said by the same theist(s), any two atheists might have very different views of the world.
That said, we can move to the second point, which is that MOST atheists are probably non or even anti-Berkeleyan, willing or even eager to asset the objective existence of matter or material substance in some sense.
A third point, though, is that yes there are SOME atheistic Berkeleyans out there. In particular, you might want to study up on Ernst Mach (1836-1916).
My understanding is that to a Machian the unobserved tree in the quad exists only as a postulated source of future or hypothetical observations. These future or hypothetical observations need not be anchored by God -- according to Machians, they float freely, as indeed do we all.
Whether you regard that as a satisfactory conception is of course up to you.