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We may sum up the matter by saying that the reality of sensible phenomena consists in the constancy of the mutual order in which they exist, and in the dependence of this order upon the divine Intelligence and Will. In this respect, at least, Leibniz resembles the young Irish idealist, Berkeley, who only seven years after Leibniz wrote the "New Essays" composed his "Principles of Human Knowledge," urging that the immediate reality of sense-phenomena consists in their "steadiness, order, and coherence," "in a constant uniform working," and that this "gives us a foresight which enables us to regulate our actions for the benefit of life." It was Berkeley also who wrote that their ultimate reality consists in their being ideas of a Divine Spirit. This was six years before the death of Leibniz. Yet it does not appear that Berkeley knew of Leibniz, and the only allusion to Berkeley which I have found in the writings of Leibniz shows that Leibniz knew only of that caricature of his views which has always been current,—that Berkeley was one who denied the existence of any external world. What he writes is as follows: "As for him in Ireland who questions the reality of 'bodies,' he seems neither to offer what is rational, nor sufficiently to explain his own ideas. I suspect that he is one of those men who are desirous of making themselves known through paradoxes."
The fundamental category of Locke, as of all who take simply a mechanical view of experience, is that of substance. He had good reason to be surprised when the Bishop of Worcester objected that Locke wished "to discard substance out of the world." How can that be so, Locke asks, when I say that "our idea of body is an extended solid substance, and our idea of soul is of a substance that thinks." And he adds, "Nay, as long as there is any simple idea or sensible quality left, according to my way of arguing, substance cannot be discarded." Everything that really exists, is, according to Locke, substance. But substance to Locke, as again to all who interpret the universe after sensible categories, is unknowable. For such categories allow only of external relations; they admit only of static existence. Substance, in this way of looking at it, must be distinct from its qualities, and must be simply the existing substratum in which they inhere.
Locke's account of the way in which we get the idea, and of its nature, is as follows: "All the ideas of all the sensible qualities of a cherry come into my mind by sensation. The ideas of these qualities and actions, or powers, are perceived by the mind to be by themselves inconsistent with existence. They cannot subsist of themselves. Hence the mind perceives their necessary connection with inherence, or with being supported." Correlative to the idea of being supported is, of course, the idea of the support. But this idea "is not represented to the mind by any clear and distinct idea; the obscure and vague, indistinct idea of thing or something, is all that is left." Or yet more simply, "Taking notice that a certain number of simple ideas go together, and not imagining how these simple ideas can subsist by themselves, we accustom ourselves to suppose some substratum wherein they do subsist, and from which they do result." Hence the only idea we have of it is of something which underlies known qualities. It is their "supposed, but unknown, support."
If we translate these expressions into the ideas of to-day, we see that they are equivalent to the view of the world which is given us by scientific categories when these categories are regarded not merely as scientific, but also as philosophic; that is, capable of interpreting and expressing the ultimate nature of experience. This modern view uses the words "things-in-themselves" (or absolute realities) and "phenomena." It says that we know nothing of existence as it is in itself, but only of its phenomena. Mind, matter, objects, are all substances, all equally substances, and all have their unknown essence and their phenomenal appearance. Such a distinction between the known and the unknown can rest, it is evident, only upon a separation between reality and phenomena similar to that which Locke makes between substance and qualities. In knowing the latter, we know nothing of the former. Although the latter are called "phenomena," they do not really manifest the substantial reality; they conceal it. This absolute distinction between substance and quality, between reality and phenomenon, rests, in turn, upon the hypothesis that reality is mere existence; that is, it is something which is, and that is all. It is a substratum; it lies under, in a passive way, qualities; it is (literally) substance; it simply stands, inactively, under phenomena. It may, by possibility, have actions; but it has them. Activities are qualities which, like all qualities, are in external relation to the substance. Being, in other words, is the primary notion, and "being" means something essentially passive and merely enduring, accidentally and secondarily something acting. Here, as elsewhere, Locke is the father of the mechanical philosophy of to-day.
We have already learned how completely Leibniz reverses this way of regarding reality. According to Locke, reality essentially is; and in its being there is no ground of revelation of itself. It then acts; but these actions, "powers, or qualities," since not flowing from the very being of substance, give no glimpse into its true nature. According to Leibniz, reality acts, and therefore is. Its being is conditioned upon its activity. It is not first there, and secondly acts; but its "being there" is its activity. Since its very substance is activity, it is impossible that it should not manifest its true nature. Its every activity is a revelation of itself. It cannot hide itself as a passive subsistence behind qualities or phenomena. It must break forth into them. On the other hand, since the qualities are not something which merely inhere in an underlying support, but are the various forms or modes of the activity which constitutes reality, they necessarily reveal it. They are its revelations. There is here no need to dwell further on the original dynamic nature of substance; what was said in the way of general exposition suffices. It is only in its relations to Locke's view as just laid down that it now concerns us.
In the first place, Leibniz points out that qualities are "abstract," while substance is "concrete." The qualities, from the very fact that they have no self-subsistence, are only relations, while the substance, as that of which they are qualities, or from which they are abstractions, is concrete. It is, Leibniz says, to invert the true order to take qualities or abstract terms as the best known and most easily comprehended, and "concretes" as unknown, and as having the most difficulty about them. "It is abstractions which give birth to almost all our difficulties," and Locke's error here is that he begins with abstractions, and takes them to be most open to intelligence. Locke's second error is separating so completely substance and attribute. "After having distinguished," says Leibniz, "two things in substance, the attributes or predicates, and the common subject of these predicates, it is not to be wondered at that we cannot conceive anything in particular in the subject. This result is necessary, since we have separated all the attributes in which there is anything definite to be conceived. Hence to demand anything more than a mere unknown somewhat in the subject, is to contradict the supposition which was made in making the abstraction and in conceiving separately the subject and its qualities or accidents." We are indeed ignorant of a subject from which abstraction has been made of all defining and characteristic qualities; "but this ignorance results from our demanding a sort of knowledge of which the object does not permit." In short, it is a credit to our knowledge, not an aspersion upon it, that we cannot know that which is thoroughly unreal,—a substance deprived of all attributes. This is, indeed, a remark which is applicable to the supposed unknowableness of pure Being, or Absolute Being, when it is defined as the absence of all relations (as is done, for example, by Mr. Spencer to-day).
