“If two things or events are essentially alike, that means that their difference may be neglected; and if they are essentially different that means that their resemblance may be neglected. Now neglect of a resemblance or difference which is really there can only be justified by reference to a purpose; its justification depends on the irrelevance of the details neglected, in regard to the special conclusion we draw. The points of likeness between the Suez and the Panama Canals, for instance, may be neglected if the points of difference are sufficient to over-ride them on the question whether the newer enterprise will pay. The importance of any resemblance or difference thus always varies with the purpose for which it is used. There is no point of resemblance or difference which is important or unimportant for all purposes at once.” (Alfred Sidgwick, 1893, The Process of Argument).