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INTRODUCTION

systematic a natural history of certain spirits of the air, on the unproven and unprovable assumption, that such creatures exist in the air, with human passions, inclinations, and conceptions, should we call such a system a science, no matter how closely its several parts might be connected with each other into a whole ? On the other hand, supposing- somebody were to utter a single proposition— a mechanic, for instance, the proposition that a pillar erected on a horizontal base in a right angle stands perpendicular, and will not incline toward either side, however far you extend it into infinity— a proposition which he may have heard at some time and approved as true in experience: would not all men concede that such a person had a scientific knowledge of the proposition, although he should not be able to evolve the deduction of his proposition from the first fundamental principle of geometry? Now, why do we call the fixed system, which rests on an unproven and unprovable first principle, no science at all, and why do we assert the knowledge of the mechanic to be science, although it does not connect in his reason with a system? Evidently, because the first, in spite of its correct form, does not contain any thing that can be known; and because the second, although without a correct form, asserts something which is really known and can be known. The characteristic of science, therefore, seems to consist in the quality of its content and the relation thereof to the consciousness of the person of whom a knowledge is