The discussion regarding “primitive truths,” axioms, and maxims, as well as the distinction between truths of fact and of reason, has its most important bearing in Locke’s next chapter. This chapter has for its title the “Extent of Human Knowledge,” and in connection with the sixth chapter, upon universal propositions, and with the seventh, upon axioms, really contains the gist of the treatment of knowledge. It is here also that are to be considered chapters three and six of book third, having respectively as their titles, “Of General Terms,” and “Of the Names of Substances.”
To understand Locke’s views upon the extent and limitations of our knowledge, it is necessary to recur to his theory of its origin. If we compare what he says about the origin of ideas from sensations with what he says about the development of general knowledge from particular, we shall find that Locke unconsciously puts side by side two different, and even contradictory, theories upon this point. In the view already given when treating of sensation, knowledge originates from the combination, the addition, of the simple ideas furnished us by our senses. It begins with the simple, the unrelated, and advances to the complex. But according to the doctrine which he propounds in treating of general terms, knowledge begins with the individual, which is already qualified by definite relations, and hence complex, and proceeds, by abstracting some of these qualities, towards the simple. Or, in Locke’s own language, “ideas become general by separating from them the circumstances of time and place and any other ideas that may determine them to this and that particular existence.” And, still more definitely, he says that general ideas are framed by “leaving out of the complex idea of individuals that which is peculiar to each, and retaining only what is common to them all.” From this it follows that “general and universal belong not to the real existence of things, but are the inventions and creatures of the understanding.” “When we quit particulars, the generals that rest are only creatures of our own making. . . . The signification they have is nothing but a relation that by the mind of man is added to them.” And in language which reminds us of Kant, but with very different bearing, he says that relations are the workmanship of the understanding. The abstract idea of what is common to all the members of the class constitutes “nominal essence.” This nominal essence, not being a particular existence in nature, but the “workmanship of the understanding,” is to be carefully distinguished from the real essence, “which is the being of anything whereby it is what it is.” This real essence is evidently equivalent to the unknown “substance” of which we have heard before. “It is the real, internal, and unknown constitution of things.” In simple or unrelated ideas and in modes the real and the nominal essence is the same; and hence whatever is demonstrated of one is demonstrated of the other. But as to substance it is different, the one being natural, the other artificial. The nominal essence always relates to sorts, or classes, and is a pattern or standard by which we classify objects. In the individual there is nothing essential, in this sense. “Particular beings, considered barely in themselves, will be found to have all their qualities equally essential to them, or, which is more, nothing at all.” As for the “real essence” which things have, “we only suppose its being without precisely knowing what it is.”
Locke here presents us with the confusion which, in one form or another, is always found in empiricism, and which indeed is essential to it. Locke, like the ordinary empiricist, has no doubt of the existence of real things. His starting-point is the existence of two substances, mind and matter; while, further, there is a great number of substances of each kind. Each mind and every separate portion of matter is a distinct substance. This supposed deliverance of common sense Locke never called into question. Working on this line, all knowledge will consist in abstraction from the ready-made things presented to us in perception, “in leaving out from the complex idea of individuals” something belonging to them. But on the other hand, Locke never doubts that knowledge begins with sensation, and that, therefore, the process of knowledge is one of adding simple, unrelated elements. The two theories are absolutely opposed to each other, and yet one and the same philosophical inference may be drawn from each; namely, that only the particular is real, and that the universal (or relations) is an artificial product, manufactured in one case by abstraction from the real individual, in the other by compounding the real sensation.
The result is, that when he comes to a discussion of the extent of knowledge, he admits knowledge of self, of God, and of “things,” only by a denial of his very definition of knowledge, while knowledge of other conceptions, like those of mathematics, is not knowledge of reality, but only of ideas which we ourselves frame. All knowledge, that is to say, is obtained only either by contradicting his own fundamental notion, or by placing it in relations which are confessedly artificial and superinduced. It is to this point that we come.
The proposition which is fundamental to the discussion is that we have knowledge only where we perceive the agreement or disagreement of ideas. Locke then takes up each of his four classes of connection, in order to ascertain the extent of knowledge in it. Our knowledge of “identity and diversity extends as far as our ideas,” because we intuitively perceive every idea to be “what it is, and different from any other.” Locke afterwards states, however, that all purely identical propositions are “trifling,” that is, they contain no instruction; they teach us nothing. Thus the first class of relations cannot be said to be of much avail. If we consider the fourth kind of knowledge, that of real existence, we have an intuitive knowledge of self, a demonstrative knowledge of God, and a sensitive knowledge of other things. But sensitive knowledge, it must be noted, “does not extend beyond the objects actually present to our senses.” It can hardly be said, therefore, to assure us of the existence of objects at all. It only tells us what experiences are being at the time undergone. Furthermore, knowledge of all three (God, self, and matter), since of real being, and not of relations between ideas, contradicts his definition of knowledge. But perhaps we shall find knowledge more extended in the other classes. And indeed Locke tells us that knowledge of relations is the “largest field of our knowledge.” It includes morals and mathematics; but it is to be noticed that, according to Locke, in both of these branches our demonstrations are not regarding facts, but regarding either “modes” framed by ourselves, or relations that are the creatures of our minds,—“extraneous and superinduced” upon the facts, as he says. He thus anticipates in substance, though not in phraseology, Hume’s distinction between “matters of fact” and “connections of ideas,” in the latter of which we may have knowledge, but not going beyond the combinations that we ourselves make.
This leaves one class, that of co-existence, to be examined. Here, if anywhere, must knowledge, worthy of being termed scientific, be found. This class, it will be remembered, comprehends our knowledge concerning substances. But this extends, 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.”