Descartes challenged philosophy to take seriously the ancient Skeptic question, “What can we know?” Because he could not trust his senses Descartes looked inside the mind to find what he could know using reason alone. He was looking for the ideas he could arrive at a priori, without (relying on unreliable) experience. He concluded, of course, that he knew, if nothing else, he existed because he cognized. Reason requires that something must exist in order to think.
Hume rejected Descartes, that there is anything that can be known without experience. Every idea necessarily has a sensuous experience from which it originated. This school of thought was called Empiricism. In additional to this, Hume rejected rational metaphysical systems that seem rational, and explain everything, but go beyond sensuous experience. A surprising example of such a metaphysical system came from a fellow Empiricist: Bishop Berkeley. Berkeley’s Idealism claimed all experience was the direct projection of God. Importantly, he believed that experiences were the source of ideas in the mind. Consequently, the world is nothing more than ideas in one’s head. Ideas such as cause and effect and the laws of nature can be explained by this metaphysical description of reality (i.e. God). Many of the philosophical problems presented by Skepticism could be resolved by Berkeley’s metaphysical description, but his metaphysical description was not immediately evident from experience itself.
Although Berkeley’s theological description of reality sounds absurd to the non-philosopher, Hume argues that more modest philosophical assumptions, and even everyday notions, such as Force, Power, and Energy rely on similar fairytales. What do we really know about the true nature of cause and effect? We could never conclude a priori that any specific effect would occur in response to a cause without prior experience. We cannot justify a definition of gravity deeper than our experience of it as an ambiguous metaphysical “force,” whatever a “force” is.
If we are being honest, the most definite we can be about cause and effect is that one event follows another. We can define the events with greater detail, and by limiting external factors, predict with increasing accuracy what events will happen under given circumstances, but we cannot claim to know a “why” beyond a description of the events. We must admit that this lack of knowledge leaves open the possibility that scientific predictions may offer surprising results.