||The best known Best Systems Account of Laws of Nature is often referred to as the Mill-Ramsey-Lewis account. All three philosophers have formulated versions of it where David Lewis's is the most sophisticated. It says, roughly: Suppose you knew everything about the past, present, and future, all facts, all events, and you organised your entire knowledge as simply as possible in various systems. The statements in these systems must mention natural properties only. A contingent generalisation is, then, a law of nature if and only if it appears as an axiom or theorem in the one system that achieves a far better combination of simplicity, strength and fit than any of the other competing systems. To have strength is to bear a great deal of informational content about the world; to be simple is to state everything in a concise way, not to be redundant, etc., to fit is, especially for the probabilistic laws, to accord as much as possible with the actual outcomes of world history.