So much for Leibniz's polemic. Its meaning is that space and time have significance only with reference to things and events, that they are the intellectual, the ideal side of these objects and occurrences, being the relations which give them order and unity. A space which is not the space of objects, which is not space in and through objects, is an inanity; it is not spirit, it is not matter; it is not a relation of either. It is nothingness magnified to infinity, and then erected into existence. And all for nothing; for it does not enable us to account for a single concrete fact of experience. For this we must have recourse to relations and orders of existence. Space is therefore to be defined as the order which makes it possible for objects to have situation; time as that which makes it possible for events to have dating,--not as if they were actually prior to them, and although nothings in themselves, yet capable of giving concrete determination to things, but as _actually_ the relations themselves, and as _ideally_ necessary for the coherent experience of co-existent objects and of connected events. As Leibniz puts it epigrammatically: "Space is the order of possible constants; time the order of inconstant possibilities." We have finished the exposition of the views of Leibniz about matter and material facts. One question, however, remains to be discussed,--a question which Leibniz's contemporary critics would not allow him to pass over in silence, even had he been so disposed. What is the reality of matter, of motion, of space, and of time? Since they are, as Leibniz says, only phenomena, not absolute realities, what distinguishes them from dreams, from illusions? What distinguishes sensible phenomena from capricious fantasies, and gives them reality?