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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?

Leibniz begins his answer by pointing out that the mere fact that bodies are phenomena does not make them unreal. To say that anything is phenomenal is to say that it is sensible; but "the senses make no declaration regarding metaphysical matters" such as truth and reality. The senses, in a word, only inform us that the experiences are there for the senses, that they are sensible. What is the ultimate nature of the sensible or the phenomenal, what is their reality, is a question wholly outside the province of sense. The questions of ultimate nature, of reality, are questions of metaphysics, and hence are to be decided by the reason, not by the senses. And Leibniz goes on to say that the truthfulness of the senses, since it concerns only the sensible, consists in the reciprocal agreement of sensible facts, and in that we are not deceived in reasoning from one to another. An isolated sense-experience could not be said to be either true or false, real or illusory. It would be true that it was experienced, and that is all that could be said about it. But since our experiences are not thus separated, but have a certain order, there arises what we may call sensible reality and illusion. When the order between two facts remains the same "in different times and places and in the experience of different men," we call these facts real. If, however, our experience cannot be repeated by ourselves or by other men when the same conditions (that is, connections) are present, it is unreal, or false. It is thus "the relation of phenomena which guarantees truth of fact regarding sensible objects." Constancy, regularity, justify us in ascribing reality; chaotic change and lack of orderly connection are a sign of unreality. Even our dreams have a reality; for they have their connections and place in experience. If we understood their connections we should even be able to explain their apparent lack of connection with the rest of experience. Leibniz thinks that both the Academicians and Sceptics and their opponents erred in attempting to find greater reality in sensible things than that of regular phenomena. Since our observations and judgments upon sensible phenomena are of such a nature that we can predict future phenomena and prepare for them, we have all the reality in them that can be had or asked for. Even if it be granted possible (as it must be on this basis) that, metaphysically speaking, sense-experience is only a connected dream, it yet has a sufficient reality; for we are not deceived in the measures taken with reference to phenomena, provided that we act on the ground of their observed harmonies and relations. Thus while we are obliged to admit that our senses inform us that there are hard, passive, extended, indivisible things, not perfectly continuous and not intellectual in their nature, and we know on metaphysical grounds that this information is not correct, we cannot say that our senses deceive us, for sense makes no statements regarding such matters. It is our reason that errs if it takes the information that the senses give as if it were a declaration of reason itself. Sensible things have all the reality necessary for this range of experience,—practical,—such regularity of co-existence and sequence as allows us to act without being led astray.

But if we regard sense-phenomena not merely in their connection with one another, but in their dependence upon the absolute realities, we have still better justification for their comparative reality. These phenomena are consequences of necessary and eternal truths. One endowed with a perfect knowledge of such truths would be able to deduce, a priori, the phenomena from them. The reality of sensible phenomena thus consists not merely in their connection with one another, but in the fact that they are connected as the laws of the intelligible world require. They follow not only rules of co-existence and sequence; but these rules may be brought under general laws of motion, which in turn may be deduced from geometrical principles. These latter, however, are a priori; they are truths which are grounded in the very intelligence of God. The sensible has its basis in the ideal. To state the same fact in another way, all sensible phenomena occur in time and space; or rather, time and space are the orders, the relations, of phenomena occurring and existing. But, as we have just seen, time and space are ideal. A relation, as Leibniz points out, being neither attribute nor accident, cannot be in the things which it relates, as their possession. In his own words, it cannot be conceived as if it had one leg in one object, the other leg in the other. A relation is not a material bond, running through or cementing objects; it is ideal, existing in the mind. And while it is true that space and time are the relations of objects and events, it is also true that if all objects and events were annihilated, space and time would continue to have their ideal existence in the intelligence of God as the eternal conditions of phenomena. They thus form the links between absolute reality and the reality of sensible existence. The principle of sufficient reason forms another link. It may be recalled that in discussing Leibniz's theory of volition we found that the will of God in relation to the sensible world is always determined by the choice of the better; that in this consists the controlling reason and regulative principle of all that occurs and exists. Thus for every fact in the sensible world there is connection with "metaphysical," or absolute, reality, not only through the medium of the intellectual relations of time and space, but through the dynamic intermediary of the divine will acting in accordance with the divine reason. Sensible facts have, then, a reality, but a dependent one. There would be no contradiction involved if they were not what they actually are.