In one sense it must be admitted that we can never prove the existence of things other than ourselves and our experiences. No logical absurdity results from the hypothesis that the world consists of myself and my thoughts and feelings and sensations, and that everything else is mere fancy. There is no logical impossibility in the supposition that the whole of life is a dream, in which we ourselves create all the objects that come before us.
Bertrand Russell – Problems of Philosophy. The Existence of Matter