Supplements are necessary for explanation wherever the immediate facts do not contain all the elements of a certain process. If an observable phenomenon has not its conditions in observable facts we hypothetically assume unobservable facts as its causes. But we may incidentally remark that description and explanation are not different in kind, but in degree. Explanation is an exhaustive description set forth in its greatest possible simplicity. An exhaustive description enumerates all the determinative factors of a process and it drops everything that is of no account, so that information is imparted with the greatest economy as well as completeness. An exhaustive description is a reliable guide to preascertain the outcome of a process, and reveals in this way the identity in the change, the continuity of the process and the conservation of matter and energy in their transformations, or, in other words, it reveals the necessity of the result. There is perhaps no natural science in which the processes can be exhaustively described without hypothetical supplements and so the science of psychology forms no exception to the general rule. The aim of psychology in its wider sense will be “to separate all the contents of consciousness into their elements, to state their laws of combination, and to seek in an empirical way for the diverse elementary psychical contents, their correspondent physiological irritations, in order to explain in this way mediately from the coexistence and succession of physiological irritations the purely psychological laws of combinations which as such are unexplainable” (p. 127).