  Probably nobody before the twentieth century ever held exactly these views without holding certain closely linked ones as well. The most common closely linked view that is associated with the traditional diagram is that the E and I propositions convert simply; that is, ‘No S is P’ is equivalent in truth value to ‘No P is S’, and ‘Some S is P’ is equivalent in truth value to ‘Some P is S’. The traditional doctrine supplemented with simple conversion is a very natural view to discuss. It is Aristotle’s view, and it was widely endorsed (or at least not challenged) before the late 19th century. I call this total body of doctrine ‘[SQUARE]’: