§ 23. The _will_ as a thing in itself is quite different from its phenomenal appearance, and entirely free from all the forms of the phenomenal, into which it first passes when it manifests itself, and which therefore only concern its _objectivity_, and are foreign to the will itself. Even the most universal form of all idea, that of being object for a subject, does not concern it; still less the forms which are subordinate to this and which collectively have their common expression in the principle of sufficient reason, to which we know that time and space belong, and consequently multiplicity also, which exists and is possible only through these. In this last regard I shall call time and space the _principium individuationis_, borrowing an expression from the old schoolmen, and I beg to draw attention to this, once for all. For it is only through the medium of time and space that what is one and the same, both according to its nature and to its concept, yet appears as different, as a multiplicity of co-existent and successive phenomena. Thus time and space are the _principium individuationis_, the subject of so many subtleties and disputes among the schoolmen, which may be found collected in Suarez (Disp. 5, Sect. 3). According to what has been said, the will as a thing-in-itself lies outside the province of the principle of sufficient reason in all its forms, and is consequently completely groundless, although all its manifestations are entirely subordinated to the principle of sufficient reason. Further, it is free from all _multiplicity_, although its manifestations in time and space are innumerable. It is itself one, though not in the sense in which an object is one, for the unity of an object can only be known in opposition to a possible multiplicity; nor yet in the sense in which a concept is one, for the unity of a concept originates only in abstraction from a multiplicity; but it is one as that which lies outside time and space, the _principium individuationis_, _i.e._, the possibility of multiplicity. Only when all this has become quite clear to us through the subsequent examination of the phenomena and different manifestations of the will, shall we fully understand the meaning of the Kantian doctrine that time, space and causality do not belong to the thing-in-itself, but are only forms of knowing.