From this essential connection between causality and succession it follows, that the conception of reciprocity, strictly speaking, has no meaning; for it presumes the effect to be again the cause of its cause: that is, that what follows is at the same time what precedes. In a "Critique of Kantian Philosophy," which I have added to my chief work, and to which I refer my readers,[64] I have shown at length that this favourite conception is inadmissible. It may be remarked, that authors usually have recourse to it just when their insight is becoming less clear, and this accounts for the frequency of its use. Nay, it is precisely when a writer comes to the end of his conceptions, that the word '_reciprocity_' presents itself more readily than any other; it may, in fact, be looked upon as a kind of alarm-gun, denoting that the author has got out of his depth. It is also worthy of remark, that the word _Wechselwirkung_, literally reciprocal action--or, as we have preferred translating it, _reciprocity_--is only found in the German language, and that there is no precise equivalent for it in daily use in any other tongue. [64] "Die Welt a. W. u. V." vol. i. pp. 517-521 of the 2nd edition, and pp. 544-549 of the 3rd edition. From the law of causality spring two corollaries which, in virtue of this origin, are accredited as cognitions _à priori_, therefore as unquestionable and without exception. They are, _the law of inertia_ and that _of permanence of substance_. The first of these laws avers, that every state in which a body can possibly be--consequently that of repose as well as that of any kind of movement--must last for ever without change, diminution, or augmentation, unless some cause supervenes to alter or annul it. But the other law, by which the eternity of Matter is affirmed, results from the fact, that the law of causality is exclusively applicable to _states_ of bodies, such as repose, movement, form, and quality, since it presides over their temporal passing in or out of being; but that it is by no means applicable to the existence of _that which endures_ these states, and is called _Substance_, in order precisely to express its exemption from all arising and perishing. '_Substance is permanent_' means, that it can neither pass into, nor out of being: so that its quantity existing in the universe can neither be increased nor diminished. That we know this _à priori_, is proved by the consciousness of unassailable certainty with which, when we see a body disappear--whether it be by conjuring, by minute subdivision, by combustion, volatilisation, or indeed any process whatever--we all nevertheless firmly assume that its substance, _i.e._ its _matter_, must still exist somewhere or other in undiminished quantity, whatever may have become of its _form_; likewise, when we perceive a body suddenly in a place, where it was not before, that it must have been brought there or formed by some combination of invisible particles--for instance, by