 These conditions are intended to serve as requirements whose satisfaction guarantees that a proposed explanation is adequate. Hempel drew several distinctions over time between potential scientific explanations (which satisfy the first three conditions, but possibly not the fourth) and confirmed scientific explanations (which are believed to be true but might turn out to be false). Hempel recognized that (CA-3) was a redundant condition, since it would have to be satisfied by any explanation that satisfied (CA-1) and (CA-2). Insofar as the explanandum describes an event that occurred during the history of the world, its derivation thereby implies the explanans has empirical content.