That the understanding, therefore, cannot make of its a priori principles, or even of its conceptions, other than an empirical use, is a proposition which leads to the most important results.Atranscendental use is made of a conception in a fundamental proposition or principle, when it is referred to things in general and considered as things in themselves; an empirical use, when it is referred merely to phenomena, that is, to objects of a possible experience.That the latter use of a conception is the only admissible one is evident from the reasons following.For every conception are requisite, firstly, the logical form of a conception (of thought)general; and, secondly, the possibility of presenting to this an object to which it may apply.Failing this latter, it has no sense, and utterly void of content, although it may contain the logical function for constructing a conception from certain data.Now, object cannot be given to a conception otherwise than by intuition, and, even if a pure intuition antecedent to the object is a priori possible, this pure intuition can itself obtain objective validity only from empirical intuition, of which it is itself but the form.All conceptions, therefore, and with them all principles, however high the degree of their a priori possibility, relate to empirical intuitions, that is, to data towards a possible experience.Without this they possess no objective validity, but are mere play of imagination or of understanding with images or notions.Let us take, for example, the conceptions of mathematics, and first in its pure intuitions."Space has three dimensions"- "Between two points there can be only one straight line," etc.Although all these principles, and the representation of the object with which this science occupies itself, are generated in the mind entirely a priori, they would nevertheless have no significance if we were not always able to exhibit their significance in and by means of phenomena (empirical objects).Hence it is requisite that an abstract conception be made sensuous, that is, that an object corresponding to it in intuition be forthcoming, otherwise the conception remains, as we say, without sense, that is, without meaning.Mathematics fulfils this requirement by the construction of the figure, which is a phenomenon evident to the senses.The same science finds support and significance in number; this in its turn finds it in the fingers, or in counters, or in lines and points.The conception itself is always produced a priori, together with the synthetical principles or formulas from such conceptions; but the proper employment of them, and their application to objects, can exist nowhere but in experience, the possibility of which, as regards its form, they contain a priori.
That this is also the case with all of the categories and the principles based upon them is evident from the fact that we cannot render intelligible the possibility of an object corresponding to them without having recourse to the conditions of sensibility, consequently, to the form of phenomena, to which, as their only proper objects, their use must therefore be confined, inasmuch as, if this condition is removed, all significance, that is, all relation to an object, disappears, and no example can be found to make it comprehensible what sort of things we ought to think under such conceptions.
The conception of quantity cannot be explained except by saying that it is the determination of a thing whereby it can be cogitated how many times one is placed in it.But this "how many times" is based upon successive repetition, consequently upon time and the synthesis of the homogeneous therein.Reality, in contradistinction to negation, can be explained only by cogitating a time which is either filled therewith or is void.If I leave out the notion of permanence (which is existence in all time), there remains in the conception of substance nothing but the logical notion of subject, a notion of which I endeavour to realize by representing to myself something that can exist only as a subject.But not only am I perfectly ignorant of any conditions under which this logical prerogative can belong to a thing, I can make nothing out of the notion, and draw no inference from it, because no object to which to apply the conception is determined, and we consequently do not know whether it has any meaning at all.
In like manner, if I leave out the notion of time, in which something follows upon some other thing in conformity with a rule, Ican find nothing in the pure category, except that there is a something of such a sort that from it a conclusion may be drawn as to the existence of some other thing.But in this case it would not only be impossible to distinguish between a cause and an effect, but, as this power to draw conclusions requires conditions of which I am quite ignorant, the conception is not determined as to the mode in which it ought to apply to an object.The so-called principle: