I published the most essential supplement to this book as early as 1836, under the title On the Will in Nature (second edition, 1854), which contains the most characteristic and important development in my philosophy, namely, the transition from the phenomenon to the thing in itself, which Kant had abandoned as impossible. It would be entirely incorrect to consider the statements of others, to which I connected my expositions in that book, as its real material and subject matter: a book that is small in volume but important as regards its content. Rather, those statements are merely what occasioned me to discuss the fundamental truth of my doctrine more distinctly there than anywhere else and trace it back to empirical cognizance of nature. And indeed this was done most exhaustively and rigorously in the chapter titled “Physical Astronomy”; so I cannot ever hope to find a more accurate and exact expression of that core of my philosophy than the one there set forth. Anyone who should want to become thoroughly acquainted with my philosophy and earnestly scrutinize it has therefore above all to take the aforementioned chapter into account. In general, then, everything that has been said in that brief work would constitute the main content of the present supplements, were it not, as having preceded them, necessary to exclude it; however, I presuppose that it is known here, in that precisely what is best would otherwise be lacking.