The result of applying the operation of variation to a proposition depends essentially on our choice of ideas to be varied in the proposition in question. And it can depend on matters of fact whether or not a proposition is universally valid (or contravalid) with respect to a sequence \(i\) of ideas. Thus, e.g., it is due to the fact that every German is European and no German is American that the proposition [Every German philosopher is European] is universally valid and the proposition [Every