Now this fundamental truth, which even to-day sounds so like a paradox, is the part of my doctrine to which, in all its chief points, the empirical sciences--themselves ever eager to steer clear of all Metaphysic--have contributed just as many confirmations forcibly elicited by the irresistible cogency of truth, but which are most surprising on account of the quarter whence they proceed; and although they have certainly come to light since the publication of my chief work, it has been quite independently of it and as the years went on. Now, that it should be precisely this fundamental doctrine of mine which has thus met with confirmation, is advantageous in two respects. First, because it is the main thought upon which my system is founded; secondly, because it is the only part of my philosophy that admits of confirmation through sciences which are alien to, and independent of, it. For although the last seventeen years, during which I have been constantly occupied with this subject, have, it is true, brought me many corroborations as to other parts, such as Ethics, Æsthetics, Dianoiology; still these, by their very nature, pass at once from the sphere of actuality, whence they arise, to that of philosophy itself: so they cannot claim to be extraneous evidence, nor can they, as collected by me, have the same irrefragable, unequivocal cogency as those concerning _Metaphysics_ proper which are given by its correlate _Physics_ (in the wide sense of the word which the Ancients gave it). For, in pursuing its own road, Physics, _i.e._, Natural Science as a whole, must in all its branches finally come to a point where physical explanation ceases. Now this is precisely the _Metaphysical_, which Natural Science only apprehends as the impassable barrier at which it stops short and henceforth abandons its subject to Metaphysics. Kant therefore was quite right in saying: "It is evident, that the primary sources of Nature's agency must absolutely belong to the sphere of Metaphysics."[189] Physical science is wont to designate this unknown, inaccessible something, at which its investigations stop short and which is taken for granted in all its explanations, by such terms as physical force, vital force, formative principle, &c. &c., which in fact mean no more than _x, y, z_. Now if nevertheless, in single, propitious instances, specially acute and observant investigators succeed in casting as it were a furtive glance behind the curtain which bounds off the domain of Natural Science, and are able not only to feel it is a barrier but, in a sense, to obtain a view of its nature and thus to peep into the metaphysical region beyond; if moreover, having acquired this privilege, they explicitly designate the limit thus explored downright as that which is stated to be the true inner essence and final principle of all things by a system of Metaphysics unknown to them, which takes its reasons from a totally