The last book of the Essay, which treats of Knowledge and Probability, is concerned more directly than the three preceding ones with Locke’s professed design. It has been suggested that Locke may have begun with this book. It contains few references to the foregoing parts of the Essay, and it might have appeared separately without being much less intelligible than it is. The other books, concerned chiefly with ideas and words, are more abstract, and may have opened gradually on his mind as he studied more closely the subject treated in the fourth book. For Locke saw that the ultimate questions about our knowledge and its extent presuppose questions about ideas. Without ideas knowledge is impossible. “Idea” is thus a leading term in the Essay. It is used in a way peculiar to himself—“the term which, I think, stands best for whatsoever is the object of the understanding when a man thinks” or “whatever it is which the mind can be employed about.” But ideas themselves are, he reminds us, “neither true nor false, being nothing but bare appearances,” phenomena as we might call them. Truth and falsehood belong only to assertions or denials concerning ideas, that is, to our interpretations of our ideas according to their mutual relations.
That none of our ideas are “innate” is the argument contained in the first book. This means that the human mind, before any ideas are present to it, is a tabula rasa: it needs the quickening of ideas to become intellectually alive. The Innate ideas. inward purpose of this famous argument is apt to be overlooked. It has been criticized as if it was a speculative controversy between empiricism and intellectualism. For this Locke himself is partly to blame. It is not easy to determine the antagonist he had in view. Lord Herbert is referred to as a defender of innateness. Locke was perhaps too little read in the literature of philosophy to do full justice to those more subtle thinkers who, from Plato downwards, have recognized the need for categories of the understanding and presuppositions of reason in the constitution of knowledge. “Innate,” Lord Shaftesbury says, “is a word Mr Locke poorly plays on.” For the real question is not about the time when ideas entered the mind, but “whether the constitution of man be such that, being adult and grown up, the ideas of order and administration of a God will not infallibly and necessarily spring up in him.” This Locke himself sometimes seems to allow. “That there are certain propositions,” we find him saying, “which, though the soul from the beginning, or when a man is born, does not know, yet, by assistance from the outward senses, and the help of some previous cultivation, it may afterwards come certainly to know the truth of, is no more than what I have affirmed in my first book” (“Epistle to Reader,” in second edition). And much of our knowledge, as he shows in the fourth book, is rational insight, immediate or else demonstrable, and thus intellectually necessary in its constitution.
What Locke really objects to is, that any of our supposed knowledge should claim immunity from free criticism. He argues in the first book against the innateness of our knowledge of God and of morality; yet in the fourth book he finds that the existence of God is demonstrable, being supported by causal necessity, without which there can be no knowledge; and he also maintains that morality is as demonstrable as pure mathematics. The positions are not inconsistent. The demonstrable rational necessity, instead of being innate, or conscious from our birth, may lie latent or subconscious in the individual mind; but for all that, when we gradually become more awake intellectually, such truths are seen to “carry their own evidence along with them.” Even in the first book he appeals to the common reason, which he calls “common sense.” “He would be thought void of common sense who asked, on the one side, or, on the other, went to give a reason, why ‘it is impossible for the same thing to be and not to be.’ It carries its own light and evidence with it, and needs no other proof: he that understands the terms assents to it for its own sake, or else nothing else will ever be able to prevail with him to do it” (bk. i. chap. 3, § 4).
The truth is that neither Locke, on the one hand, nor the intellectualists of the 17th century, on the other, expressed their meaning with enough of precision; if they had, Locke’s argument would probably have taken a form less open to the charge of mere empiricism. Locke believed that in attacking “innate principles” he was pleading for universal reasonableness instead of blind reliance on authority, and was thus, as he says, not “pulling up the foundations of knowledge,” but “laying those foundations surer.” When men heard that there were propositions that could not be doubted, it was a short and easy way to assume that what are only arbitrary prejudices are “innate” certainties, and therefore must be accepted unconditionally. This “eased the lazy from the pains of search, stopped the inquiry of the doubtful, concerning all that was once styled innate. It was no small advantage to those who affected to be masters and teachers to make this the principle of principles—that principles must not be questioned.” The assumption that they were “innate” was enough “to take men off the use of their own reason and judgment, and to put them upon believing and taking upon trust without further examination.... Nor is it a small power it gives one man over another to have the authority to make a man swallow that for an innate principle which may serve his purpose who teacheth them” (bk. i. chap. 4, § 24).
The second book proposes a hypothesis regarding the genesis of our ideas and closes after an elaborate endeavour to verify it. The hypothesis is, that all human ideas, even the most complex and abstract and sublime, ultimately depend upon Genesis of ideas. “experience.” Otherwise, what we take to be ideas are only empty words. Here the important point is what human “experience” involves. Locke says that our “ideas” all come, either from the five senses or from reflective consciousness; and he proposes to show that even those concerned with the Infinite depend at last on one or other of these two sources: our “complex ideas” are all made up of “simple ideas,” either from without or from within. The “verification” of this hypothesis, offered in the thirteenth and following chapters of the second book, goes to show in detail that even those ideas which are “most abstruse,” how remote soever they may seem from original data of outward sense, or of inner consciousness, “are only such as the understanding frames to itself by repeating and joining together simple ideas that it had at first, either from perceiving objects of sense, or from reflection upon its own operations.”
To prove this, our thoughts of space, time, infinity, power, substance, personal identity, causality, and others which “seem most remote from the supposed original” are examined in a “plain historical method,” and shown to depend either on (a) perception of things external, through the five senses, or on (b) reflection upon operations of the mind within. Reflection, “though it be not sense, as having nothing to do with external objects,” is yet, he says, “very like it, and might properly enough be called internal sense.” But the suggestion that “sense” might designate both the springs of experience is misleading, when we find in the sequel how much Locke tacitly credits “reflection” with. The ambiguity of his language makes opposite interpretations of this cardinal part of the Essay possible; the best we can do is to compare one part with another, and in doubtful cases to give him the benefit of the doubt.
Although the second book is a sort of inventory of our ideas, as distinguished from the certainty and boundaries of our knowledge, Locke even here makes the assumption that the “simple ideas” of the five senses are practically qualities of things which exist without us, and that the mental “operations” discovered by “reflection” are those of a person continuously existing. He thus relieves himself of the difficulty of having at the outset to explain how the immediate data of outward sense and reflection are accepted as “qualities” of things and persons. He takes this as a fact.
Such, according to Locke, are the only simple ideas which can appear even in the sublimest human speculations. But the mind, in becoming gradually stored with its “simple ideas” is able to elaborate them in numberless modes and relations; although it is not in the power of the most exalted wit or enlarged understanding to invent or frame any new simple idea, not taken in in one or the other of these two ways. All that man can imagine about the universe or about God is necessarily confined to them. For proof of this Locke would have any one try to fancy a taste which had never affected his palate, or to frame the idea of a scent he had never felt, or an operation of mind, divine or human, foreign to all human consciousness.
The contrast and correlation of these two data of experience is suggested in the chapter on the “qualities of matter” in which we are introduced to a noteworthy vein of speculation (bk. ii. chap. 8). This chapter, on “things and their Qualities of matter. qualities,” looks like an interpolation in an analysis of mere “ideas.” Locke here treats simple ideas of the five senses as qualities of outward things. And the sense data are, he finds, partly (a) revelations of external things themselves in their mathematical relations, and partly (b) sensations, boundless in variety, which are somehow awakened in us through contact and collision with things relatively to their mathematical relations. Locke calls the former sort “primary, original or essential qualities of matter,” and the others “secondary or derived qualities.” The primary, which are quantities rather than qualities, are inseparable from matter, and virtually identical with the ideas we have of them. On the other hand, there is nothing perceived in the mathematical relations of bodies which in the least resembles their secondary qualities. If there were no sentient beings in existence, the secondary qualities would cease to exist, “except perhaps as unknown modes of the primary, or, if not, as something still more obscure.” On the other hand, “solidity, extension, figure and motion would,” he assumes, “be really in the world as they are, whether there were any sensible being to perceive them or not.”
Thus far the outcome of what Locke teaches about matter is, that it is Something capable of being expressed in terms of mathematical quantity, and also in terms of our own sensations. A further step was to suggest the ultimate dependence of the Matter. secondary qualities of bodies upon “the bulk, figures, number, situation and motions of the solid parts of which the bodies consist,” these mathematical or primary qualities “existing as we think of them whether or not they are perceived.” This Locke proposes in a hesitating way. For we, “not knowing what particular size, figure and texture of parts they are on which depend, and from which result, those qualities which make our complex idea, for example, of gold, it is impossible we should know what other qualities result from, or are incompatible with, the same constitution of the insensible parts of gold; and so consequently must always coexist with that complex idea we have of it, or else are inconsistent with it.”