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 precipitation--but that it, _i.e._ its substance, cannot have then started into existence; for this implies a total impossibility and is utterly inconceivable. The certainty with which we assume this beforehand (_à priori_), proceeds from the fact, that our Understanding possesses absolutely no form under which to conceive the beginning and end of Matter. For, as before said, the law of causality--the only form in which we are able to conceive changes at all--is solely applicable to _states_ of bodies, and never under any circumstances to the existence of _that which undergoes_ all changes: _Matter_. This is why I place the principle of the permanence of Matter among the corollaries of the causal law. Moreover, we cannot have acquired _à posteriori_ the conviction that substance is permanent, partly because it cannot, in most instances, be empirically established; partly also, because every empirical knowledge obtained exclusively by means of induction, has only approximate, consequently precarious, never unconditioned, certainty. The firmness of our persuasion as to this principle is therefore of a different kind and nature from our security of conviction with regard to the accuracy of any _empirically_ discovered law of Nature, since it has an entirely different, perfectly unshakable, never vacillating firmness. The reason of this is, that the principle expresses a _transcendental_ knowledge, _i.e._ one which determines and fixes, _prior_ to all experience, what is in any way possible within the whole range of experience; but, precisely by this, it reduces the world of experience to a mere cerebral phenomenon. Even the most universal among the non-transcendental laws of Nature and the one least liable to exception--the law of gravitation--is of empirical origin, consequently without guarantee as to its absolute universality; wherefore it is still from time to time called in question, and doubts occasionally arise as to its validity beyond our solar system; and astronomers carefully call attention to any indications corroborative of its doubtfulness with which they may happen to meet, thereby showing that they regard it as merely empirical. The question may of course be raised, whether gravitation takes effect between bodies which are separated by an _absolute_ vacuum, or whether its action within a solar system may not be mediated by some sort of ether, and may not cease altogether between fixed stars; but these questions only admit of an empirical solution, and this proves that here we have not to do with a knowledge _à priori_.