George Berkeley (1685-1753) is best known for arguing for Idealism on empiricist grounds. In metaphysics, Idealism is the view that there is no physical substance underlying our sense impressions of the world. Rather, the world consists entirely of ideas. Your mind is just a bundle of impressions, and there is nothing in the world except for so many minds having their various perceptions. Berkeley defends this as the view that best accords with common sense in Three Dialogues between Hylas and Philonous.
Berkeley’s argument attacks Locke’s distinction between primary and secondary qualities and argues that all of our sense impressions are mere appearances and that we have no grounds for thinking that any of them bear any resemblance to the way things are. Since we lack any empirical experience of the underlying substances in which qualities inhere, we have no empirical reason to suppose underlying substances even exist. All we have access to are our sense impressions, and these are mental things, ideas. So all we can claim knowledge of are our ideas beginning with our sense impressions, the most basic ideas.
Berkeley also argues that positing underlying substances do no significant explanatory work. So, the common sense empiricist view ought to be that we live in a world of ideas that lacks any underlying physical substance. This startling view might make us wonder what happens to my desk when I leave the room and cease to perceive it. Does it pop out of existence when I leave and then pop back into existence just as it was when I return to my work? This would be most peculiar. Berkeley argues that the objects of our everyday life do have an enduring existence when we are absent. They continue to exist as ideas in the mind of God. Given this appeal to the mind of God to explain the continued existence of things we aren’t actively observing, we might argue that positing underlying substances does some explanatory work after all and charge that Berkeley has only substituted one unobservable theoretical posit, God, for another, underlying substances.