That therefore is true of sciences in general which we are apt to think applies only to pure mathematics, though to pure mathematics it applies especially, viz., that they cannot be considered as simple representations or informants of things as they are. We are accustomed to say, and say truly, that the conclusions of pure mathematics are applied, corrected, and adapted, by mixed; but so too the conclusions of Anatomy, Chemistry, Dynamics, and other sciences, are revised and completed by each other. Those several conclusions do not represent whole and substantive things, but views, true, so far as they go; and in order to ascertain how far they do go, that is, how far they correspond to the object to which they belong, we must compare them with the views taken out of that object by other sciences. Did we proceed upon the abstract theory of forces, we should assign a much more ample range to a projectile than in fact the resistance of the air allows it to accomplish. Let, however, that resistance be made the subject of scientific analysis, and then we shall have a new science, assisting, and to a certain point completing, for the benefit of questions of fact, the science of projection. On the other hand, the science of projection itself, considered as belonging to the forces it contemplates, is not more perfect, as such, by this supplementary investigation. And in like manner, as regards the whole circle of sciences, one corrects another for purposes of fact, and one without the other cannot dogmatize, except hypothetically and upon its own abstract principles. For instance, the Newtonian philosophy requires the admission of certain metaphysical postulates, if it is to be more than a theory or an hypothesis; as, for instance, that what happened yesterday will happen to-morrow; that there is such a thing as matter, that our senses are trustworthy, that there is a logic of induction, and so on. Now to Newton metaphysicians grant all that he asks; but, if so be, they may not prove equally accommodating to another who asks something else, and then all his most logical conclusions in the science of physics would remain hopelessly on the stocks, though finished, and never could be launched into the sphere of fact.