George Berkeley
Berkeley argued that forces and gravity, as defined by Newton, constituted "occult qualities" that "expressed nothing distinctly". He held that those who posited "something unknown in a body of which they have no idea and which they call the principle of motion, are in fact simply stating that the principle of motion is unknown." Therefore, those who "affirm that active force, action, and the principle of motion are really in bodies are adopting an opinion not based on experience." Forces and gravity existed nowhere in the phenomenal world. On the other hand, if they resided in the category of "soul" or "incorporeal thing", they "do not properly belong to physics" as a matter. Berkeley thus concluded that forces lay beyond any kind of empirical observation and could not be a part of proper science. He proposed his theory of signs as a means to explain motion and matter without reference to the "occult qualities" of force and gravity.
The Force
Astrophysicist Jeanne Cavelos says in "The Science of Star Wars" that the Force raises questions that scientists have long asked. She points out that the ancient Greeks explored the idea of a "fifth element" that permeated and connected everything in the universe, and that Isaac Newton proposed that the human brain might be able to trigger waves in the ether, giving humans psychic powers. Cavelos's sources are mostly skeptical about a "real world" explanation for the Force, but they explore ideas in areas such as quantum physics, parapsychology, and the notion of science so advanced that it appears magical. Explaining the Force is particularly difficult, Cavelos says, because "it does so many different things". Several scientists have said it is best not to try to explain how the Force works.