In nature, which consists of ideas of perception, everything that happens is necessary; for it proceeds from its cause. If, however, we consider this individual with reference to everything else which is not its cause, we know it as contingent; but this is already an abstract reflection. Now, further, let us abstract entirely from a natural object its causal relation to everything else, thus its necessity and its contingency; then this kind of knowledge comprehends the conception of the _actual_, in which one only considers the _effect_, without looking for the cause, in relation to which one would otherwise have to call it _necessary_, and in relation to everything else _contingent_. All this rests ultimately upon the fact that the _modality_ of the judgment does not indicate so much the objective nature of things as the relation of our knowledge to them. Since, however, in nature everything proceeds from a cause, everything _actual_ is also _necessary_, yet only so far as it is _at this time, in this place;_ for only so far does determination by the law of causality extend. Let us leave, however, concrete nature and pass over to abstract thinking; then we can present to ourselves in reflection all the natural laws which are known to us partly _a priori_, partly only _a posteriori_, and this abstract idea contains all that is in nature at _any_ time, in _any_ place, but with abstraction from every definite time and place; and just in this way, through such reflection, we have entered the wide kingdom of _the possible_. But what finds no place even here is the _impossible_. It is clear that possibility and impossibility exist only for reflection, for abstract knowledge of the reason, not for knowledge of perception; although it is the pure forms of perception which supply the reason with the determination of the possible and impossible. According as the laws of nature, from which we start in the thought of the possible and impossible, are known _a priori_ or _a posteriori_, is the possibility or impossibility metaphysical or physical. From this exposition, which requires no proof because it rests directly upon the knowledge of the principle of sufficient reason and upon the development of the conceptions of the necessary, the actual, and the possible, it is sufficiently evident how entirely groundless is Kant’s assumption of three special functions of the understanding for these three conceptions, and that here again he has allowed himself to be disturbed by no reflection in the carrying out of his architectonic symmetry.