John Stuart Mill had a theory of perception, which is commonly referred to as classical phenomenalism. This differs from Berkeley's idealism in its account of how objects continue to exist when no one is perceiving them. Berkeley claimed that an omniscient God perceived all objects and this is what kept them in existence, whereas Mill claimed that permanent possibilities of experience were sufficient for an object's existence. These permanent possibilities could be analyzed into subjunctive conditionals, such as, if I were to have y-type sensations, then I would also have x-type sensations.
Phenomenalism and the bundle theory