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It is true, however, that he occasionally uses the terms as synonymous; but this where the distinction between them has no bearing on the argument in hand, and where the context determines in what sense the term is used. The distinction which he actually makes, and to which he keeps when space and time are the subject of discussion, is that extension and duration are qualities or predicates of objects and events, while space and time are relations, or orders of existence. Extension and duration are, as he says, the immensity, the mass, the continuation, the repetition, of some underlying subject. But space and time are the measure of the mass, the rule or law of the continuation, the order or mode of the repetition. Thus immediately after the passage already quoted, in which he says that extension in body is the diffusion of materiality, just as whiteness is the diffusion of a property of milk, he goes on to say "that extension is to space as duration to time. Duration and extension are attributes of things; but space and time are to be considered, as it were, outside of things, and as serving to measure them." Still more definitely he says: "Many confound the immensity or extent of things with the space by means of which this extent is defined. Space is not the extension of body, any more than duration is its time. Things keep their extension, not always their space. Everything has its own extent and duration; but it does not have a time of its own, nor keep for its own a space." Or, as he expresses the latter idea elsewhere, space is like number, in the sense that it is indifferent to spatial things, just as number is indifferent to res numerata. Just as the number five is not a quality or possession of any object, or group of objects, but expresses an order or relation among them, so a given space is not the property of a thing, but expresses the order of its parts to one another. But extension, on the other hand, is a property of the given objects. While extension, therefore, must always belong to some actual thing, space, as a relation, is as applicable to possible things as to actual existences; so that Leibniz sometimes says that time and space "express possibilities." They are that which makes it possible for a definite and coherent order of experiences to exist. They determine existence in some of its relations, and as such are logically prior to any given forms of existence; while extent and duration are always qualities of some given form of existence, and hence logically derivative. Since time and space "characterize possibilities" as well as actualities, it follows as a matter of course "that they are of the nature of eternal truths, which relate equally to the possible and to the existing." Being an eternal truth, space must have its place in that which is simply the active unity of all eternal truths,—the mind of God. "Its truth and reality are based upon God. It is an order whose source is God." Since God is purus actus, he is the immediate, the efficient source only of that which partakes in some degree of his own nature, or is rational; and here is another clear point of distinction between space and extension, between time and duration.

But we must ask more in detail regarding their nature. Admitting that they are relations, ideal and prior to particular experiences, the question must be asked, What sort of relations are they; how are they connected with the purely spiritual on one hand, and with the phenomenal on the other? Leibniz's most extended answers to these questions are given in his controversy with Clarke. The latter took much the same position regarding the nature of space (though not, indeed, concerning the origin of its idea) as Locke, and the arguments which Leibniz uses against him he might also have used, for the most part, against Locke. Locke and Clarke both conceived of space and time as wholly without intrinsic relation to objects and events. It is especially against this position that Leibniz argues, holding that space and time are simply orders or relations of objects and events, that space exists only where objects are existing, and that it is the order of their co-existence, or of their possible co-existence; while time exists only as events are occurring, and is the relation of their succession. Clarke, on the other hand, speaks of the universe of objects as bounded by and moving about in an empty space, and says that time existed before God created the finite world, so that the world came into a time already there to receive its on-goings, just as it fell into a space already there to receive its co-existences.

To get at the ideas of Leibniz, therefore, we cannot do better than follow the course of this discussion. He begins by saying that both space and time are purely relative, one being the order of co-existences, the other of successions. Space characterizes in terms of possibility an order of things existing at the same time, so far as they exist in mutual relations (ensemble), without regard to their special modes of existence. As to the alternate doctrine that space is a substance, or something absolute, it contradicts the principle of sufficient reason. Were space something absolutely uniform, without things placed in it, there would be no difference between one part and another, and it would be a matter of utter indifference to God why he gave bodies certain positions in space rather than others; similarly it would be a matter of indifference why he created the world when he did, if time were something independent of events. In other words, the supposed absoluteness of space and time would render the action of God wholly without reason, capricious, and at haphazard. Similarly, it contradicts the principle of "indiscernibles," by which Leibniz means the principle of specification, or distinction. According to him, to suppose two things exactly alike, is simply to imagine the same thing twice. Absolute uniformity, wholly undifferentiated, is a fiction impossible to realize in thought. "Space considered without objects has nothing in it to determine it; it is accordingly nothing actual. The parts of space must be determined and distinguished by the objects which are in them." Finally, were space and time absolutely real things in themselves, they would be independent of God, and even limitations upon him. "They would be more substantial than substances. God would not be able to change or destroy them. They would be immutable and eternal in every part. Thus there would be an infinity of eternal things (these parts) independent of God." They would limit God because he would be obliged to exist in them. Only by existing through this independent time would he be eternal; only by extending through this independent space would he be omnipresent. Space and time thus become gods themselves.

When Clarke declares that by the absoluteness of space and time he does not mean that they are themselves substances, but only properties, attributes of substance, Leibniz advances the same arguments in different form. If space were the property of the things that are in space, it would belong now to one substance, now to another, and when empty of all material substance, even to an immaterial substance, perhaps to God. "Truly a strange attribute which is handed about from one thing to another. Substances thus leave their accidents as if they were old clothes, and other substances put them on." Since these finite spaces are in infinite space, and the latter is an attribute of God, it must be that an attribute of God is composed of parts, some of them empty, some full, some round, some square. So, too, whatever is in time would help make one of the attributes of God. "Truly a strange God," says Leibniz, "this Deity of parts" (ce Dieu parties). Clarke's reply to this was that space and time are attributes of God and of God alone, not of things in space and time,—that, indeed, strictly speaking, there are no parts in space or in time; they are absolutely one. This was virtually to give up the whole matter. It was to deny the existence of finite spaces and times, and to resolve them into an indefinite attribute of God. Such a view, as Leibniz points out, not only is contrary to experience, but affords no aid in determining the actual concrete forms and situations of bodies, and durations and successions of events. The absolute space and time, having no parts, are wholly out of relations to these concrete existences. The latter require, therefore, a space and a time that are relations or orders. Clarke's hypothesis is, as Leibniz says, wholly without use or function, and requires a theory like that of Leibniz to account for the actually determinate forms of experience. In his last reply Clarke shifts his ground again, and says that space and time are effects of God's existence; "they are the necessary results of his existence." "His existence is the cause of space and time." The death of Leibniz prevented any further reply. It is not hard to imagine, however, that in a general way his reply would have been to ask how space and time are at once attributes essential and necessary to God, as constituting his immensity and eternity, and effects dependent upon his existence. To take this latter position, indeed, seems to abandon the position that they are absolute, and to admit that, like the rest of God's creation, they are relative and finite.