Essentially the Same

“What we do, whenever we argue by analogy from case S to case S, is to class these two cases together, though only for a special and limited purpose. We are, or profess to be, sufficiently aware of all the differences that exist between, say, the postal service and the railway service, but we claim that for the one purpose of satisfactory management in the public interest these differences are negligible. When we say that any two things are essentially the same we never mean that they are precisely similar in every respect; but that, though different, their difference is unimportant as compared with their resemblance.” (Alfred Sidgwick, The Application of Logic)