according to Locke, “a very little way.” The idea of a substance is a complex of various “simple ideas united in one subject and co-existing together.” When we would know anything further concerning a substance, we only inquire what other simple ideas, besides those already united, co-exist with them. Since there is no necessary connection, however, among these simple ideas, since each is, by its very simplicity, essentially distinct from every other, or, as we have already learned, since nothing is essential to an individual, we can never be sure that any idea really co-exists with others. Or, as Locke says, in physical matters we “can go no further than particular experience informs us of. . . . We can have no certain knowledge of universal truths concerning natural bodies.” And again, “universal propositions of whose truth and falsehood we have certain knowledge concern not existence;” while, on the other hand, “particular affirmations are only concerning existence, declaring only the accidental union or separation of ideas in things existing.” This particular knowledge, it must be recalled, is, in turn, only sensitive, and thus extends not beyond the time when the sensation is had.
We are not surprised then at learning from Locke that regarding bodies “we are not capable of scientific knowledge.” “Natural philosophy is not capable of being made a science;” or, as Locke elsewhere states it, knowledge regarding the nominal essence is “trifling” (Kant’s analytic judgment); regarding the real essence is impossible. For example, when we say that all gold is fusible, this means either simply that fusibility is one of the ideas which we combine to get the general idea of gold, so that in making the given judgment we only expand our own notion; or it means that the “real” substance gold is always fusible. But this is a statement we have no right to make, and for two reasons: we do not know what the real substance gold is; and even if we did, we should not know that fusibility always co-exists with it. The summary of the whole matter is that “general certainty is to be found only in our ideas. Whenever we go to seek it elsewhere, in experiment or observations without us, our knowledge goes not beyond particulars.”
It has been necessary to give an account of Locke’s views at this length because it is in his discussion of the limitations and extent of knowledge that his theory culminates. While not working out his sensationalism as consistently as did Hume, he yet reduces knowledge to that of the existence of God and ourselves (whose natures, however, are unknown), and to a knowledge of mathematical and moral relations, which, however, concerns only “the habitudes and relations of abstract ideas.” We have now to see by what means Leibniz finds a wider sphere for certain and general knowledge by his theory of intellectualism than Locke can by his sensationalism.
Leibniz’s theory of knowledge rests upon a distinction between truths of fact, which are a posteriori and contingent, and truths of reason, which are a priori and necessary. In discussing his views regarding experience, we learned that, according to him, all judgments which are empirical are also particular, not allowing any inference beyond the given cases experienced. Experience gives only instances, not principles. If we postpone for the present the discussions of truths of reason, by admitting that they may properly be said to be at once certain and universal, the question arises how in matters of fact there can be any knowledge beyond that which Locke admits; and the answer is, that so far as the mere existence and occurrence of these facts is concerned, there is neither demonstrative nor general knowledge. But the intelligence of man does not stop with the isolated fact; it proceeds to inquire into its cause, to ascertain its conditions, and thus to see into, not merely its actual existence, but its possibility. In Leibniz’s language: “The real existence of things that are not necessary is a point of fact or history; but the knowledge of possibilities or necessities (the necessary being that whose opposite is not possible) constitutes demonstrative science.” In other words, it is the principle of causality, which makes us see a fact not as a mere fact, but as a dependent consequence; which elevates knowledge, otherwise contingent and particular, into the realm of the universal and apodictic. Underlying all “accidental union” is the real synthesis of causation.
If we follow the discussion as it centres about the terms “nominal” and “real,” it stands as follows: Leibniz objects to the use of the term “essence” in this connection, but is willing to accept that of “definition;” for, as he says, a substance can have but one essence, while there may be several definitions, which, however, all express the same essence. The essence is the possibility of that which is under consideration; the definition is the statement of that which is supposed to be possible. The “nominal” definition, however, while it implies this possibility, does not expressly affirm it,—that is to say, it may always be doubted whether the nominal definition has any possibility (or reality) corresponding to it until experience comes to our aid and makes us know it a posteriori. A “real” definition, on the other hand, makes us know a priori the reality of the thing defined by showing us the mode of its production, “by exhibiting its cause or generation.” Even our knowledge of facts of experience cannot be said, therefore, to be arbitrary, for we do not combine ideas just as we please, but “our combinations may be justified by reason which shows them to be possible, or by experience which shows them to be actual, and consequently also possible.” To take Locke’s example about gold, “the essence of gold is that which constitutes it and gives it its sensible qualities, and these qualities, so far as they enable us to recognize it, constitute its nominal essence, while a real and causal definition would enable us to explain the contexture or internal disposition. The nominal definition, however, is also real in one sense,—not in itself, indeed, since it does not enable us to know a priori the possibility or production of the body, but empirically real.”
It is evident from these quotations that what Leibniz understands by “possibility” is the condition or cause of a given fact; and that, while Locke distinguishes between particular, accidental and demonstrative, general knowledge as two opposed kinds, concerned with two distinct and mutually exclusive spheres, with Leibniz they are distinctions in the aspect of the same sphere of fact. In reality there is no combination of qualities accidental, as Locke thought that by far the greater part were; in every empirical fact there is a cause or condition involved that is invariable, and that constitutes the reason of the fact. The “accidental” is only in the relation of our ideas to objects, not in the objects themselves. There may be accidental mental associations; there are no accidental relations. In empirical, or a posteriori, knowledge, so-called, the reason is there, but is not known. A priori knowledge, the real definition, discovers and explicitly states this reason. Contingent knowledge is therefore potentially rational; demonstrative knowledge is the actual development of the reasons implicitly contained in experience.
We may with advantage connect this discussion with the fundamental doctrine of Locke and Leibniz regarding intelligence and reality. To Locke, as we have seen, knowledge is essentially a matter of relations or connections; but relations are “superinduced” and “extraneous” as regards the facts. Every act of knowledge constitutes, therefore, in some way a departure from the reality to be known. Knowledge and fact are, by their very definition, opposed to one another. But in Leibniz’s view intelligence, or reason, enters into the constitution of reality; indeed, it is reality. The relations which are the “creatures of the understanding” are, therefore, not foreign to the material to be known, but are organic to it, forming its content. The process, then, in which the mind perceives the connections or relations of ideas or objects, is simply the process by which the mind comes to the consciousness of the real nature of these objects, not a process of “superinducing” unreal ideas upon them. The difficulty of Locke is the difficulty of every theory of knowledge that does not admit an organic unity of the knowing mind and the known universe. The theory is obliged to admit that all knowledge is in the form of relations which have their source in intelligence. But being tied to the view that reality is distinct from intelligence, it is obliged to draw the conclusion that these relations are not to be found in actual existence, and hence that all knowledge, whatever else it may be, is unreal in the sense that it does not and cannot conform to actual fact. But, in the theory of Leibniz, the process of relating which is the essence of knowledge is only the realization on the part of the individual mind of the relations or reasons that eternally constitute reality. Since reality is,
