Ether is, in effect, a merely hypothetical entity, valuable only in so far as it explains that which by means of it we endeavor to explain — light, electricity, or universal gravitation — and only so far as these facts cannot be explained in any other way. In like manner the idea of God is also an hypothesis, valuable only in so far as it enables us to explain that which by means of it we endeavor to explain — the essence and existence of the Universe — and only so long as these cannot be explained in any other way. And since in reality we explain the Universe neither better nor worse with this idea than without it, the idea of God, the supreme petitio principii, is valueless.

The Tragic Sense of Life (1913) - VIII : From God to God