Would it for instance be in conformity with the nature of the thing, to cut away, as you wish to do, only that group of economic facts which relates to objects capable of measurement? What intrinsic connection is there between this merely accidental attribute, measurability, of the objects which enter into an economic action, and the economic action itself? Does measurability lead to a modification in the economic fact by changing its nature, _i.e._ by giving rise to _another_ fact? If so, you must prove it. I, for my part, cannot see that an economic action changes its nature whether it relates to a sack of potatoes, or consists in an exchange of protestations of affection! In your reply you refer to the need of avoiding waste of time over matters that are too simple, for which 'it is not worth while to set in motion the great machine of mathematical reasoning.' But this need relates to the pedagogy of the professional chair or of the book, not to the science in itself, which alone we are now discussing. It is quite evident that anybody who speaks or writes lays more stress on those portions which he thinks harder for his hearers and readers to grasp, or more useful to be told. But he who thinks, _i.e._ speaks with himself, pays attention to all portions without preferences and without omissions. We are now concerned with thought, that is with the _growth_ of science; not with the manner of _communicating_ it. And in thought, we cannot admit arbitrary judgments. Nor need we be turned aside by an analogy with the _classes_ of facts, made by zoology and other natural sciences. The classifications of zoology and botany are not scientific operations, but merely views in perspective; and, considered in relation to really scientific knowledge, they are arbitrary. He who investigates the nature of economic data, does not, however, aim at putting together, in perspective and roughly, groups of economic cases, as the zoologist or the botanist do, mutilating and manipulating the inexhaustible, infinite varieties of living creatures. Upon the confusion between _a science_ and the _exposition of a science_ is based also the belief that we can follow different paths in order to arrive at a demonstration of the same truth. Unless in your case, since you are a mathematician, it arose from a false analogy with calculation. Now, calculation is not a science, because it does not give us the reasons of things; and hence _mathematical_ logic is logic in a manner of speaking, a variety of formal logic, and has nothing to do with _scientific_ or inventive logic.