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 firs...