Some of the most remarkable chapters in the second book concern what may be called “crucial instances” in verification of its fundamental hypothesis of the dependence of human knowledge upon the simple ideas presented in our dual experience (bk. ii. ch. 13-28). They carry us towards the ultimate mysteries which attract meditative minds. The hypothesis, that even our most profound and sublime speculations are all limited to data of the senses and of reflection, is crucially tested by the “modes” and “substances” and “relations” under which, in various degrees of complexity, we somehow find ourselves obliged to conceive those simple phenomena. Such are modes of quantity in space, and time and number, under which Locke reports that we find ourselves mentally impelled towards immensity, eternity and the innumerable—in a word, towards Infinity which seems to transcend quantity; then there is the complex thought of Substance, to which we find ourselves mysteriously impelled, when the simple phenomena of the senses come to be regarded as qualities of “something”; again there is the obscure idea of the identity of persons, notwithstanding their constant changes of state; and there is, above all, the inevitable tendency we somehow have to refund a change into what we call its “Cause,” with the associated idea of active power. Locke begins with our complex ideas of Space, Succession or Time, and Number.
Space, he says, appears when we use our senses of sight and touch; succession he finds “suggested” by all the changing phenomena of sense, and by “what passes in our minds”; number is “suggested by every object of our senses, and Immensity and endlessness and infinity. every thought of our minds, by everything that either doth exist or can be imagined.” The modifications of which these are susceptible he reports to be “inexhaustible and truly infinite, extension alone affording a boundless field to the mathematicians.” But the mystery latent in our ideas of space and time is, that “something in the mind” irresistibly hinders us from allowing the possibility of any limit to either. We find ourselves, when we try, compelled to lose our positive ideas of finite spaces in the negative idea of Immensity or Boundlessness, and our positive ideas of finite times in the negative thought of Endlessness. We have never seen, and we cannot imagine, an object whose extent is boundless. Yet we find when we reflect that something forces us to think that space and time must be unlimited. Thus Locke seems by implication to acknowledge something added by the mind to the original “simple ideas” of extension and succession; though he finds that what is added is not positively conceivable. When we reflect on immensity and eternity, we find them negations of all that is imaginable; and that whether we try infinite addition or infinite subdivision. He accepts this fact; he does not inquire why mind finds itself obliged to add without limit and to divide without limit. He simply reports that immensity and eternity are inevitable negative ideas, and also that every endeavour to realize them in positive images must be an attempt to represent as quantity what is beyond quantity. After all our additions we are as far from the infinite idea as we were at the beginning.
Locke is too faithful to facts to overlook the ultimate mysteries in human experience. This is further illustrated in his acknowledgment of the inconceivable that is at the root of our idea of Substance. He tries to phenomenalize it, and thus resolve Substance and personality. it into simple ideas; but he finds that it cannot be phenomenalized, and yet that we cannot dispense with it. An unsubstantiated succession of phenomena, without a centre of unity to which they are referable as qualities, is unintelligible: we cannot have a language of adjectives without nouns. Locke had some apprehension of this transcendent intellectual obligation. According to his report, “the mind” always obliges us to suppose Something beyond positive phenomena to which the phenomena must be attributed; but he was perplexed by this “confused negative” idea. So for him the word substance means “only an uncertain supposition of we know not what.” If one were to ask him what the substance is in which this colour and that taste or smell inhere, “he would find himself in a difficulty like that of the Indian, who, after saying that the world rested on an elephant, and the elephant on a broad-backed tortoise, could only suppose the tortoise to rest on ‘Something, I know not what.’” The attempt to conceive it is like the attempt positively to conceive immensity or eternity: we are involved in an endless, ultimately incomprehensible, regress. We fail when we try either positively to phenomenalize substance or to dispense with the superphenomenal abstraction. Our only positive idea is of an aggregate of phenomena. And it is only thus, he says, that we can approach a positive conception of God, namely by “enlarging indefinitely some of the simple ideas we received from reflection.” Why man must remain in this mental predicament, Locke did not inquire. He only reported the fact. He likewise struggled bravely to be faithful to fact in his report of the state in which we find ourselves when we try to conceive continued personal identity. The paradoxes in which he here gets involved illustrate this (bk. ii. ch. 27).
Locke’s thoughts about Causality and Active Power are especially noteworthy, for he rests our knowledge of God and of the external universe on those ultimate ideas. The intellectual demand for “the cause” of an event is what we find we cannot help Causality. having; yet it is a demand for what in the end the mind cannot fully grasp. Locke is content to trace the idea of “cause and effect,” as far as mere natural science goes, to our “constant observation” that “qualities and finite substances begin to exist, and receive their existence from other beings which produce them.” We find that this connexion is what gives intelligibility to ceaseless and what seemed chaotic changes, converting them into the divinely concatenated system which we call “the universe.” Locke seems hardly to realize all that is implied in scientific prevision or expectation of change. Anything, as far as “constant observation” tells us, might a priori have been the natural cause of anything; and no finite number of “observed” sequences, per se, can guarantee universality and necessity. The idea of power, or active causation, on the other hand, “is got,” he acknowledges, not through the senses, but “through our consciousness of our own voluntary agency, and therefore through reflection” (bk. ii. ch. 21). In bodies we observe no active agency, only a sustained natural order in the succession of passive sensuous phenomena. The true source of change in the material world must be analogous to what we are conscious of when we exert volition. Locke here unconsciously approaches the spiritual view of active power in the physical universe afterwards taken by Berkeley, forming the constructive principle of his philosophy.
Locke’s book about Ideas leads naturally to his Third Book which is concerned with Words, or the sensible signs of ideas. Here he analyses “abstract ideas,” and instructively illustrates the confusion apt to be produced in them by the inevitable Ideas and words. imperfection of words. He unfolds the relations between verbal signs and the several sorts of ideas; words being the means for enabling us to treat ideas as typical, abstract and general. “Some parts of this third book,” concerning Words, Locke tells his friend Molyneux, “though the thoughts were easy and clear enough, yet cost me more pains to express than all the rest of my Essay. And therefore I should not much wonder, if there be in some places of it obscurity and doubtfulness.”
The Fourth Book, about Knowledge proper and Probability, closes the Essay. Knowledge, he says, is perception of relations among ideas; it is expressed in our affirmations and negations; and real knowledge is discernment of the Theory of knowledge. relations of ideas to what is real. In the foregoing part of the Essay he had dealt with “ideas” and “simple apprehension,” here he is concerned with intuitive “judgment” and demonstrative “reasoning,” also with judgments and reasonings about matters of fact. At the end of this patient search among our ideas, he supposes the reader apt to complain that he has been “all this while only building a castle in the air,” and to ask what the purpose of all this stir is, if we are not thereby carried beyond mere ideas. “If it be true that knowledge lies only in the agreement or disagreement of ideas, the visions of an enthusiast and the reasonings of a sober man will be equally certain. It is no matter how things themselves are” (bk. iv. 4). This gives the keynote to the fourth book. It does not, however, carry him into a critical analysis of the rational constitution of knowledge, like Kant. Hume had not yet shown the sceptical objections against conclusions which Locke accepted without criticism. The subtle agnostic, who doubted reason because reason could not be supported in the end by empirical evidence, was less in his view than persons blindly resting on authority or prejudice. Total scepticism he would probably have regarded as unworthy of the serious attention of a wise man. “Where we perceive the agreement or disagreement of any of our ideas there is certain knowledge; and wherever we are sure these ideas agree with the reality of things, there is certain real knowledge” (bk. iv. ch. 4).
Locke’s report about human knowledge and its narrow extent forms the first thirteen chapters of the fourth book. The remainder of the book is concerned for the most part with the probabilities on which human life practically turns, as he and Butler are fond of Four sorts of knowable relations. reminding us. As regards kinds of knowledge, he finds that “all knowledge we are capable of” must be assertion or denial of some one of three sorts of relation among our ideas themselves, or else of relations between our ideas and reality that exists independently of us and our ideas. Accordingly, knowledge is concerned either with (a) relations of identity and difference among ideas, as when we say that “blue is not yellow”; or (b) with mathematical relations, as that “two triangles upon equal bases between two parallels must be equal”; or (c) in assertions that one quality does or does not coexist with another in the same substance, as that “iron is susceptible of magnetical impressions, or that ice is not hot”; or (d) with ontological reality, independent of our perceptions, as that “God exists” or “I exist” or “the universe exists.” The first sort is analytical; mathematical and ethical knowledge represents the second; physical science forms the third; real knowledge of self, God and the world constitutes the fourth.
Locke found important differences in the way in which knowledge of any sort is reached. In some instances the known relation is self-evident, as when we judge intuitively that a circle cannot be a triangle, or that three must be more than two. In Intuition and demonstration. other cases the known relation is perceived to be intellectually necessary through the medium of premisses, as in a mathematical demonstration. All that is strictly knowledge is reached in these two ways. But there is a third sort, namely sense-perception, which hardly deserves the name. For “our perceptions of the particular existence of finite beings without us” go beyond mere probability, yet they are not purely rational. There is nothing self-contradictory in the supposition that our perceptions of things external are illusions, although we are somehow unable to doubt them. We find ourselves inevitably “conscious of a different sort of perception,” when we actually see the sun by day and when we only imagine the sun at night.
Locke next inquired to what extent knowledge—in the way either of intuitive certainty, demonstrative certainty, or sense perception—is possible, in regard to each of the four (already mentioned) sorts of knowable relation. There is only one of the four in which our knowledge is co-extensive with our ideas. It is that of “identity and diversity”: we cannot be conscious at all without distinguishing, and every affirmation necessarily implies negation. The second sort of knowable relation is sometimes intuitively and sometimes demonstrably discernible. Morality, Locke thinks, as well as mathematical quantity, is capable of being demonstrated. “Where there is no property there is no injustice,” is an example of a proposition “as certain as any demonstration in Euclid.” Only we are more apt to be biassed, and thus to leave reason in abeyance, in dealing with questions of morality than in dealing with problems in mathematics.
Turning from abstract mathematical and moral relations to concrete relations of coexistence and succession among phenomena—the third sort of knowable relation—Locke finds the light of pure reason disappear; although these relations form “the greatest and most important part of what we desire to know.” Of these, including as they do all inductive science, he reports that demonstrable knowledge “is very short, if indeed we have any at all”; and are not thrown wholly on presumptions of probability, or else left in ignorance. Man cannot attain perfect and infallible science of bodies. For natural science depends, he thinks, on knowledge of the relations between their secondary qualities on the one hand, and the mathematical qualities of their atoms on the other, or else “on something yet more remote from our comprehension.” Now, as perception of these atoms and their relations is beyond us, we must be satisfied with inductive presumptions, for which “experimental verification” affords, after all, only conclusions that wider experience may prove to be inadequate. But this moral venture Locke accepts as “sufficient for our purposes.”