Notwithstanding statements to the contrary in the text, the book is probably quite intelligible in itself, apart from the treatise “On the Fourfold Root of the Principle of Sufficient Reason.” It has, however, been considered desirable to add an abstract of the latter work in an appendix to the third volume of this translation. R. B. H. J. K. PREFACE TO THE FIRST EDITION. I propose to point out here how this book must be read in order to be thoroughly understood. By means of it I only intend to impart a single thought. Yet, notwithstanding all my endeavours, I could find no shorter way of imparting it than this whole book. I hold this thought to be that which has very long been sought for under the name of philosophy, and the discovery of which is therefore regarded by those who are familiar with history as quite as impossible as the discovery of the philosopher’s stone, although it was already said by Pliny: _Quam multa fieri non posse, priusquam sint facta, judicantur?_ (Hist. nat. 7, 1.) According as we consider the different aspects of this one thought which I am about to impart, it exhibits itself as that which we call metaphysics, that which we call ethics, and that which we call æsthetics; and certainly it must be all this if it is what I have already acknowledged I take it to be. A _system of thought_ must always have an architectonic connection or coherence, that is, a connection in which one part always supports the other, though the latter does not support the former, in which ultimately the foundation supports all the rest without being supported by it, and the apex is supported without supporting. On the other hand, a _single thought_, however comprehensive it may be, must preserve the most perfect unity. If it admits of being broken up into parts to facilitate its communication, the connection of these parts must yet be organic, _i.e._, it must be a connection in which every part supports the whole just as much as it is supported by it, a connection in which there is no first and no last, in which the whole thought gains distinctness through every part, and even the smallest part cannot be completely understood unless the whole has already been grasped. A book, however, must always have a first and a last line, and in this respect will always remain very unlike an organism, however like one its content may be: thus form and matter are here in contradiction.