In these sentences Hume has sufficiently condemned the vulgar objections brought against abstract reasoning. Deep and difficult questions can be discussed in no other manner; and what is often called a popular treatise on some subject of philosophic inquiry can never be more than a statement of its writer's opinions, or possibly of his sentimental prejudices. The next paragraph contains Hume's earliest[48] sketch of that critical inquiry into Human Nature on which he proposed to base all future philosophy. It is of course deeply interesting. "'Tis evident that all the sciences have a relation, greater or less, to human nature; and that however wide any of them may seem to run from it, they still return back by one passage or another. Even _Mathematics_, _Natural Philosophy_, and _Natural Religion_, are in some measure dependent on the science of MAN; since they lie under the cognizance of men, and are judged of by their powers and faculties. "'Tis impossible to tell what changes and improvements we might make in these sciences were we thoroughly acquainted with the extent and force of human understanding, and could explain the nature of the ideas we employ, and of the operations we perform in our reasonings. And these improvements are the more to be hoped for in natural religion, as it is not content with instructing us in the nature of superior powers, but carries its views further, to their disposition towards us, and our duties towards them; and consequently we ourselves are not only the beings that reason, but also one of the objects concerning which we reason." "If, therefore, the sciences of mathematics, natural philosophy, and natural religion, have such a dependence on the knowledge of man, what may be expected in the other sciences, whose connection with human nature is more close and intimate?... In these four sciences of _Logic_, _Morals_, _Criticism_, and _Politics_, is comprehended almost everything which it can any way import us to be acquainted with, or which can tend either to the improvement or ornament of the human mind.