The way in which we are led to regard human knowledge is like the way in which Copernicus was led to regard the heavens. When the explanation of the celestial motions could not be made to go right so long as he assumed that all the host of stars turns round the spectator, he tried whether it would not answer better if he made the spectator turn, and left the stars at rest. We may make a similar trial in Metaphysics, as to our way of looking at objects. If our view of them must be governed altogether by the properties of the objects themselves, I see not how man can know anything about them _à priori_. But if the thing, as an object of the senses, is regulated by the constitution of our power of knowing, I can very readily represent to myself this possibility. KANT, _Kritik d. R. V. Pref._ {{87}} BOOK II. THE PHILOSOPHY OF THE PURE SCIENCES. [The principal question discussed in the last Book was this (see chaps. V. and VI.): How are _necessary_ and _universal_ truths possible? And the answer then given was: that the necessity and universality of truths are derived from the _Fundamental Ideas_ which they involve. And we proceed in this Book to exemplify this doctrine in the case of the truths of Geometry and Arithmetic, which derive their necessity and universality from the Fundamental Ideas of Space, and Time, or Number. The question thus examined is that which Kant undertook to deal with in his celebrated work, _Kritik der reinen Vernunft_ (_Examination of the Pure Reason_): and our solution of the Problem, so far as the Ideas of Space and Time are concerned, agrees in the main with his. The arguments contained in chapters II. and **VII. of this Book, are the leading arguments respecting Space and Time, in Kant's _Kritik_. Kant, however, instead of calling Space and Time _Ideas_, calls them the necessary _Forms_ of our experience, as I have stated in the text. But though I have adopted Kant's arguments as to Space and Time, all that follows in the succeeding Books, with regard to other Ideas, has no resemblance to any doctrines of Kant or his school (with the exception, perhaps, of some of the views on the Idea of _Cause_). The nature and character of the other Scientific Ideas which I have examined, in the succeeding Books, have been established by an analysis of the history of the several Sciences to which those Ideas are essential, and an examination of the writings of the principal discoverers in those Sciences.] {{88}} CHAPTER I. OF THE PURE SCIENCES.