Locke described his purpose in the Essay Concerning Human Understanding as being "to inquire into the origin, certainty, and extent of human knowledge, together with the grounds and degrees of belief, opinion, and assent" (Book I, Chapter 1, Section 1). To carry out his purpose, Locke investigated extensively "the nature of the understanding," in order to discover its powers: "how far they reach, to what things they are in any degree proportionate, and where they fail us" (Book I, Chapter 1, Section 4).
Locke's hope was that an investigation of the powers of the human understanding would reveal how we ought to govern the formation of the opinions on which our action is based. He was especially concerned to avoid skepticism. If we stray beyond the proper reach of our understanding, we find no opinion that is not subject to dispute, and this in turn might lead us to "question everything and disclaim all knowledge, because some things are not to be understood" (Book I, Chapter 1, Section 4).
In the "Epistle to the Reader" that serves as a preface to the Essay, Locke contrasts his task with that of the "master-builders, whose mighty designs, in advancing the sciences, will leave lasting monuments to the admiration of posterity." Among these "master-builders" is "the incomparable Mr. Newton." Locke takes it to be "ambition enough to be employed as an under-laborer in clearing the ground a little, and removing some of the rubbish that lies in the way to knowledge." Whereas the "master-builders" erected theories of nature, Locke hoped to correct the abuse of language he thought had covered up ignorance and hindered true knowledge. "To break in upon the sanctuary of vanity and ignorance will be, I suppose, some service to human understanding."
So how do we go about investigating the powers of our understanding? Locke thought it best to employ a "historical, plain method" (Introduction, Section 2). He contrasts his method with metaphysical "speculations" about the essence of the mind, the relation of the mind to the body, etc. Instead, he will focus on "the discerning faculties of a man as they are employed about the objects with which they have to do" (Introduction, Section 2).
Following Descartes, Locke calls the objects with which our mental faculties have to do "ideas," which are "whatever is the object of the understanding when a man thinks" (Introduction, Section 8). The first two books of the Essay are devoted to an examination of ideas, and the results of the final two books are based on the outcome of that examination, so "ideas" play a central role in the Essay.
One of Locke's most important critics, Edward Stillingfleet, Bishop of Worcester, charged that Locke's "new way of ideas" is useful for critics of Christianity (Discourse, in Vindication of the Doctrine of the Trinity). He thought of it as a fad, commenting that "every age must have its new modes" (Quoted by Locke in the Second Letter to Stillingfleet). Locke unapologetically defended his use of the term.
So that my way of ideas, and of coming to certainty by them, is to employ our minds in thinking upon something; and I do not see but your lordship yourself, and every body else, must make use of my way of ideas, unless they can find out a way that will bring them to certainty by thinking on nothing. (First Letter to Stillingfleet)If we grant that an idea is what we think about (in the broadest sense of "think"), it would seem that any object is an idea insofar as it is thought about. A table, a tree, God, all are objects of thought and in this respect they are "ideas." This is not the end of the story, however, because as we will see, Locke introduces a distinction in Chapter 8, Section 7, of Book II, which greatly complicates his "way of ideas."
To clear the way for his own account of knowledge, Locke attacks a rival view, according to which we have at least some knowledge in the form of "innate principles." Locke gives as examples, "Whatever is, is" and "It is impossible for the same thing to be and not to be" (Book I, Chapter 2, Section 4). Locke was attacking specifically the doctrine that innate principles are "some primary notions, characters, as it were, stamped upon the mind of man, which the soul receives in its very first being and brings into the world with it" (Book I, Chapter 2, Section 1). It is not clear exactly who Locke supposed to hold such a view, though a likely target is Descartes, who explicitly claimed that we have innate ideas.
Locke's general attitude is that innateness is not required for our knowledge of the truth of these principles, since "we may observe in ourselves faculties fit to attain as easy and certain knowledge of them as if they were originally imprinted on the mind" (Book I, Chapter 2, Section 1). We will see at the end of this section how knowledge of these principles supposed to occur. But Locke acknowledges that an alternative explanation is not proof that there are no innate principles, so he tries to eliminate every possible reason for thinking that there are.
Locke examines three pieces of evidence for the innateness of principles:
Children and idiots, Locke tells us, do not assent to the propositions that are supposed to be innate. The problem is that they do not perceive these propositions and do not understand them. The principles are supposed to be "imprinted" on the mind, as this is the only way to account for their innateness. But anything that is imprinted on the mind is known to the mind. "No proposition can be said to be in the mind which it never yet knew, which it was never yet conscious of" (Book I, Chapter 2, Section 5).
Locke bases his claim on the consequences of denying it. Suppose there can be unknown propositions in the mind. This could only mean that the mind is capable of knowing it, and all true propositions are so capable. In that case, all propositions would be known innately.
So that if the capacity of knowing is the natural impression contended for, all the truths a man ever comes to know will, by this account, be every one of them innate; and this great point will amount to no more, but only to a very improper way of speaking, which, while it pretends to assert the contrary, says nothing different from those who deny innate principles. (Book I, Chapter 2, Section 5)Locke drives home the point by implying that to call unknown propositions innate is an abuse of language.
For if these words (to be in the understanding) have any propriety, they signify to be understood, so that to be in the understanding and not to be understood, to be in the mind and never to be perceived, is all one as to say anything is and is not in the mind or the understanding. (Book I, Chapter 2, Section 5)An important consequence of this view is that ideas, that is, objects of thought or perception, cannot be in the understanding without being thought or perceived.
Locke next turns his attention to a weaker claim, that some principles are assented to by anyone with the use of reason. This would exclude children and idiots and so be safe from Locke's counter-examples. Locke's first response is that it is not the case that everyone knows these principles to be true when they first attain the use of reason. "How many instances of the use of reason we may observe in children a long time before they have any knowledge of this maxim, 'That it is impossible for the same thing to be and not to be'" (Book I, Chapter 2, Section 12)?
If it be objected that the principles become known through the use of reason, Locke has a ready response. On this view, whatever we discover through the use of reason would have to be innate, which is contrary to the innateness doctrine. Further, reason is a faculty of inference, not a faculty for the discovery of truths directly.
The third piece of evidence for innateness is that people assent to certain propositions as soon as they are proposed to them and understood by them. The problem here is that too many principles would have to be counted as innate, including specific propositions about numbers or high-level scientific propositions such as that two bodies cannot be in the same place (at the same time). A further argument is that in order for a principle to be innate, the ideas making it up must be innate. But then the innateness of "sweetness is not bitterness," which is known as soon as proposed and understood, would require that the ideas of sweetness and bitterness be innate, which no one would allow.
In the Preface to New Essays Concerning Human Understanding, Leibniz tries to undermine Locke's rejection of innate ideas and principles. He attacks a key assumption behind Locke's reasoning, that "Our able author seems to claim that there is nothing potential in us, and even nothing that we are not always actually conscious of perceiving." Leibniz sees no reason why there may not be a source of innate ideas deep in our souls. Playing on Locke's description of the mind as initially a blank tablet (see the next section), Leibniz notes that no tablet is so blank as not to have some features of its own. Innate principles may lie in our minds in the same way that the rough shape of a statue may exist in the veins of a piece of marble. "This is how ideas and truts are innate in us, as natural inclinations, dispositions, habits, or potentialites are, and not as actions are."
In Section 15 of Chapter 2, Locke gives the alternative account of these principles that was mentioned above. The process begins with the population of the "yet empty cabinet" of the mind with ideas, which get names attached to them. Eventually, we abstract from our particular ideas and form general ideas, to which general names are also attached. We then observe that some general ideas agree with one another, and some do not. On this basis, we know the truths of general propositions expressing these agreements or disagreements. In this way a child can come to know, for example, that sweetness is not bitterness.
According to Locke, ideas are the product of experience. Locke claims that the mind begins as "white paper" and is filled up entirely through experience.
From where does it come by that vast store which the busy and boundless fancy of man has painted on it with an almost endless variety? From where does it have all the materials of knowledge? To this I answer in one word, from experience; our knowledge is founded in all that, and from that it ultimately derives itself. (Book II, Chapter 1, Section 2)Thus Locke is very clearly an "empiricist," in that he finds in experience the origin of all objects of the human mind. Experience provides ideas of "external sensible objects" and "the internal operations of our own minds," and nothing more. So these objects and operations are "the fountains of knowledge from which all the ideas we have, or can naturally have, do spring" (Book II, Chapter 1, Section 2). The first kind of ideas Locke will call "ideas of sensation," the second "ideas of reflection." Ideas of sensation are historically the first, since the mind can begin to operate only when it gets an initial supply of ideas, and we can have ideas of reflection only when there are mental operations upon which to reflect.
Locke described experience itself based on a definite conception of human nature. He thought of human beings as having been created by God with bodies which are "fitted" to receive information about external objects through the senses (Book II, Chapter 1, Section 24). He also accepted the atomists' description of the physical world as consisting of tiny corpuscles which bring about change through their motion. (This ancient view was championed by Descartes's critic Pierre Gassendi in France and Locke's Royal Society colleague Robert Boyle in England.) Although he did not venture to describe how the process takes place, Locke thought that ideas of sensation are the product of the activity of corpuscles, through which the human senses interact with the sensible objects.
The effect of the body's relation to sensible objects is the production of "impressions that are made on our senses by objects that are extrinsic to the mind" (Book II, Chapter 1, Section 24). These impressions in turn give rise to "the perception of those ideas that are annexed to them" (Book II, Chapter 1, Section 24). In Section 17 of Chapter 11, Locke compares the understanding to a "dark room," whose only "windows" which let in "light" are external and internal sensation. "For I think the understanding is not much unlike a closet wholly shut from light, with only some little openings left, to let in external visible resemblances or ideas of things without."
Now we must face an interpretive issue of great importance. Locke has defined "idea" as what our thoughts are about. We can now say that ideas are product of two kinds of thinking: sensing and reflecting. The objects of sensing are "external," while the objects of reflecting are "internal." This makes it seem as if ideas of sensation are the external objects themselves, or perhaps are these objects as they are perceived by the mind. (For example, an idea might be a table, or the part of the table that I can see.)
On the other hand, Locke's account of perception suggests that ideas of sensation are themselves "internal," or "in the mind." They are mental entities which are the end-result of the process of sensation. A mental process of "taking notice" of the impression is required for perception. For example, someone who is deep in contemplation may not notice "impressions of sounding bodies made upon the organ of hearing" (Book II, Chapter 9, Section 4). More generally, what ordinarily tends
to produce the idea, though conveyed in by the usual organ, not being taken notice of in the understanding and so imprinting no idea in the mind, there follows no sensation--so that wherever there is sense or perception, there some idea is actually produced and present in the understanding. (Book II, Chapter 9, Section 4)It seems, then, that Locke is operating with a notion of "idea" which might be taken to refer to "external sensible objects," or taken to refer to the internal products of external sensible objects.
Locke recognized this ambiguity and admitted it candidly. He states that he will use the word "idea" ambiguously to refer both to the "immediate object of perception," which the mind "perceives in itself," and to the quality producing the idea (in the strict sense): "which ideas if I speak of sometimes as in the things themselves, I should be understood to mean those qualities in the objects which produce them in us" (Book II, Chapter 8, Section 8). Throughout the remainder of the Essay, Locke maintains this ambiguous usage.
The importance of making this distinction can be seen from Locke's reply to Stillingfleet quoted above. Following the "way of ideas" would be uncontroversial if "idea" were taken in the most general sense, in which a table or a quality of a table might be an idea. But Locke's "way of ideas" in most cases treats ideas as what is perceived "in the mind," and the purely mental status of these ideas gives rise to skeptical problems--some to be found in Locke himself, and others advanced by Berkeley and Hume.
Having described the origins of ideas, Locke turned his attention to the ideas themselves. He described certain ideas as being simple. Simple ideas are "uncompounded appearances" (Book II, Chapter 2, Section 1). These homogeneous ideas, Locke added, are not of our own making and not subject to destruction by any act of our mind. Lockean simple ideas resemble very much the corpuscles of Boyle and the atomists, and they have come to be known as "psychological atoms."
Simple ideas are classified in the following way. (The list is partial.)
In Chapter 8, Locke introduces the notion of a quality. The first definition of a quality is that it is a power in a "subject" to produce an idea in the mind. Some bodies operate directly on our minds to produce ideas, as when, say, a hot piece of wax is touched. Other bodies operate indirectly on our minds, as does the sun which heats the wax that is hot to the touch. This much is quite straightforward. But as we have seen, Locke sometimes signifies the qualities in the object in using the word "idea," which makes his discussion somewhat more difficult to follow.
Qualities of bodies are initially divided into two classes: primary and secondary. In two places, Locke describes the primary qualities as belonging to the "solid parts" of bodies (Book II, Chapter 8, Sections 23 and 26). Thus it might be best to turn to Locke's discussion of solidity in Chapter 4 to get some insight into what in Chapter 8 he calls "primary qualities."
Solidity is "That which . . . hinders the approach of two bodies, when they are moved one towards the other" (Book II, Chapter 4, Section 1). Locke tells us that solidity "of all other, seems the idea [quality] most intimately connected with, and essential to body" (Book II, Chapter 4, Section 1). All "masses of matter" which are sensible bodies are solid, and the mind projects this quality down to the "minutest particle of matter that can exist" (Book II, Chapter 4, Section 1) In this way, the mind finds solidity "inseparably inherent in body, wherever or however modified" (Book II, Chapter 4, Section 1).
In Chapter 4, Section 1, Locke also mentions that figure as well as solidity can be "traced down" to the smallest material particles. Chapter 8 adds three more qualities of this kind: bulk (or "extension"), mobility, and number. Taken together, then, these are the "primary qualities" of bodies, which are
such as are utterly inseparable from the body in whatever state it is; such as in all the alterations and changes it suffers, all the force can be used upon it, it constantly keeps; and such as sense constantly finds in every particle of matter which has bulk enough to be perceived; and the mind finds inseparable from every particle of matter.In Locke's example, a single body, a grain of wheat, may be divided into two or more smaller bodies, but each of these, taken as a body, has all the primary qualities. The number of each of the smaller particles individually is one, and taken together is a specific quantity greater than one.
It seems that we must understand Locke as saying that each body has all the primary qualities in some manner or other. (Descartes would call the manner of existence of qualities "modes," but Locke uses that expression differently.) In one place, Locke refers to the "modifications" of primary qualities (Book II, Chapter 8, Section 23). Consider figure. The specific shape of a body may change if its smaller parts are re-arranged. But the body does not lose the quality of having a figure.
Locke sometimes calls the primary qualities "real qualities." "The particular bulk, number, figure, and motion of the parts of fire or snow are really in them;" they "really exist in those bodies" (Book II, Chapter 8, Section 17). He further held that the (immediate) ideas we have of primary qualities resemble the qualities themselves (Book II, Chapter 8, Section 18).
The basis of Locke's claim seems to be this. The primary qualities of bodies are inseparable from them because we cannot conceive bodies to exist without them. We conceive of bodies through our ideas of figure, bulk, solidity, etc., so if there are bodies, they are figured, bulky, solid, and so on. If a thing did not conform to ideas that are inseparable from it, it would not be a thing. Take as an example figure, which exists in a specific body, a piece of manna. Locke asserts that "a circle or square are the same, whether in idea or existence, either in the mind or in the manna" (Book II, Chapter 8, Section 18). Thus, if we conceive the manna to be round, the roundness in our conception is the same as the roundness in an existing thing.
With this description of primary qualities in hand, we may move to secondary qualities. One way to understand them is by contrast to primary qualities: they are not inseparable from bodies, i.e., we can conceive of bodies which do not have them. Bodies only have secondary qualities insofar as they stand in a relation to a perceiver. Color is a secondary quality, and Locke contended that a body has no color when it is in the dark (Book II, Chapter 8, Section 19).
According to Locke, all of our sensible ideas are produced by the action of insensible particles of matter on our physical senses. This interaction in turn is based on the configuration of the corpuscles making up the body which is sensibly perceived. Each corpuscle in the configuration has all the primary qualities. It is through these qualities that they are able to produce ideas of the body.
Some of the ideas of the body depend essentially on the character of the senses upon which the bodies make their impressions. The eye gets impressions of colors, the ear of sounds, the nose of smells, the tongue of tastes. From these impressions ideas are formed. These are ideas of secondary qualities.
A secondary quality itself is a power in the body to produce these ideas through the activities of its insensible parts, which "operate after a peculiar manner on any of our senses, and thereby produce in us the different ideas of several colors, sounds, smells, tastes, etc" (Book II, Chapter 8, Section 23).
So there are two senses in which secondary qualities are secondary. First, they depend entirely on primary qualities, being powers of bodies which are based on their primary qualities, and second, as necessarily depending on the body's relation to a sensible organ, rather than being a characteristic of the body taken by itself. The general outlines of the primacy of some qualities over others can be found in the ancient atomists as well as in Descartes, Galileo and Boyle.
Locke adduced various arguments to show that the idea of heat (and the other ideas of secondary qualities) does not resemble the power in the body that produced it. Two of them are well-known and will later play into the hands of Berkeley. The first is that ideas of secondary qualities are relative. Air feels cool before a workout and warm afterwards, for example. Yet there was no change in the air itself. The only thing that changed was the state of the body. So the ideas of heat and cold do not resemble something in the air itself.
The second argument is that heat and cold (as well as the other secondary qualities) are uniquely associated with pleasure and pain. It is acknowledged by everyone that our ideas of pleasure and pain do not resemble anything in bodies. But heat can change insensibly into pain when it increases. Thus the heat we feel is not in body we call "hot."
A third kind of quality (sometimes called "tertiary") is a power to produce an idea indirectly, by producing a change in a body which in turn produces an idea in the mind. So the primary qualities of the sun have the power to produce changes in the positions and motions of the corpuscles making up a piece of wax, which then give us an idea of heat (Book II, Chapter 8, Section 23).
Perhaps the most important simple idea is that of power. As we have seen, Locke regards all qualities as powers of one kind or another, so the idea of power seems to be even more fundamental than the idea of any of the primary qualities. This idea is discussed in Book II, Chapter 21, which is the longest chapter of the Essay.
The idea of power arises when we observe a pattern of repetition. A quality of a certain kind is continually followed by another quality of a certain kind. Patterns of succession are found among sensible objects and in the mind itself. The mind then concludes "from what it has so constantly observed to have been, that the like changes will for the future be made in the same things, by like agents, and by the like ways" (Book II, Chapter 21, Section 1). From this, we get an idea of power, which is the possibility of changing a simple idea (active power) and the possiblity of a simple idea being changed (passive power).
It seems on the face of it that power is a complex idea, because it relates one thing to another (the cause to the effect). Locke has two reasons to think that the idea is simple. First, all the other ideas he classifies as "simple" have some "secret relation in them" (Book II, Chapter 21, Section 3). Examples are the ideas of motion (change of relative position) and figure (the parts of which stand in relation to one another). The second reason is that it is a "principal ingredient in our complex ideas of substances" (Book II, Chapter 21, Section 3).
Locke writes as if we can observe the power itself. The mind observes changes "sometimes by the impression of outward objects on the senses and sometimes by the determination of its own choice" (Book II, Chapter 21, Section 1). But Hume will ask how it is that we can have an idea of an "impression" or a "determination," since all we really observe is a succession.
Locke thought that minds afford the best idea of active power, since we have no notion of how bodies are able to initiate changes in other bodies. Since bodies only passively communicate motion, "this gives us a very obscure idea of an active power of moving in a body, while we observe it only to transfer but not produce any motion" (Section 4). On the other hand, "we find by experience that barely by willing it, barely by a thought of the mind, we can move the parts of our bodies which were before at rest" (Book II, Chapter 21, Section 4). We also can observe the production of thoughts in our minds by willing that we think them.
The power to produce motion in the body and thoughts in the mind is what Locke called will. So the will is an active power. Individual volitions or acts of will occur when the mind exercises its power. Philosophers have busied themselves with the question of whether the will is free, but Locke believed this is a misguided question, given his account of will. Freedom is a characteristic of a mind, in that one is able "to think or not to think, to move or not to move, according to the preference or direction of his own mind" (Book II, Chapter 21, Section 8). So to attribute freedom to the will would be to attribute a power to a power. Locke described this in terms of what we now call (after Gilbert Ryle) a "category mistake," akin to saying that sleep is swift or virtue is square.
Locke understood human freedom as the ability of the mind to bring about change based entirely on what it prefers to be the case. So if a person cannot move his arm because it is chained to a wall, the person is not free in this respect. Now it may be that the person in chains has no desire or preference to move his arm at all. This does not imply that he is freely at rest, since he could not execute the order if it suited his preferences. Thus, what is voluntary, i.e., what conforms to one's preferences, does not always coincide with what one is free to do.
We have seen that Locke held it to be a mistake to think that individual acts of will are free. His detailed account of willing shows more concretely why this is so. First, our understanding proposes an action to our thoughts. Our automatic response is to prefer to act or to refrain from action. If we prefer to act, we shall invariably do so.
For it is unavoidably necessary to prefer the doing or abstaining of an action in a man's power which is once so proposed to his thoughts. A man must necessarily will the one or the other of them, upon which preference or volition the action or its abstaining certainly follow and is truly voluntary. But the act of volition, or preferring one of the two, being that which he cannot avoid, a man, in respect of his willing, is under a necesssity, and so cannot be free, unless necessity and freedom can consist together and a man can be free and bound at once. (Book II, Chapter 21, Section 23)This account shows that the freedom of a human being is of a pretty weak variety. It is really only the absence of hindrances to the carrying out of our preferences. But our preferences themselves are determined by our present state of uneasiness. We will prefer to act only when we feel that the present situation is too uncomfortable to avoid action. Human freedom does not rest in the choice of preferences. Instead, it shows itself in our rational deliberations. Suppose there is a situation that we wish to change. We may suspend any action designed to bring about the change in order to evaluate how best to relieve our discomfort with the status quo.
This, it seems to me, is the great privilege of finite intellectual beings; and I desire it may be well-considered whether the great inlet and exercise of all the liberty men have, are capable of, or can be useful to them, and that on which depends the turn of their actions, does not lie in this, that they can suspend their desires, and stop them from determining their wills to any action, until they have fully and fairly examined the good and evil of it, insofar as the weight of the thing requires. (Book II, Chapter 21, Section 52)
Simple ideas are the building-blocks of complex ideas, which are made voluntarily by the mind. Locke describes three processes for the formation of complex ideas:
The only kind of complex ideas we will discuss here are those of substances. These ideas are combinations of simple ideas that are presumed to belong together so as to make up one thing. We create names for these ideas, such as "a pig" or "a ship," and we are tempted to think these names stand for simple ideas rather than combinations.
Ideas of substances may be specific or general. The idea of a specific substance is the idea of a concrete, individual thing such as the mind of John Locke, or of the mind of Robert Boyle. When we say that they are both substances, we are led to ask what they have in common which makes them both substances. Something they share is the power to think and the power to act. But these are not what moves us to call them substances; rather, it is because the ideas of these powers are presumed to go together to make up a single thing--a mind.
Now we are led to ask why it is that we combine these powers into the idea a single thing. Locke says that the reason is we have an idea of "substance in general," which is a "support" of the powers or qualities that specific substances have. Our notion of substance in general is only a relative one, as we cannot say anything more about substance in general except that it is whatever unites powers or qualities into one thing. If asked what it is that supports the powers and qualities of a thing, one can only reply that it is "something, I know not what." This purely relative conception of substance would come under strong attack from George Berkeley.
In Book III, Chapter 6 of the Essay, Locke described another way of treating our ideas of substances. We rank individual substances into kinds: this nugget and that ring are both gold. We ascribe to them an essence, some powers or qualities without which they would not be the kind of things they are. This allows us to divide the world up into what we now call "natural kinds." For example, gold is a metal with a distinctive yellowish color, which can be pounded into different shapes or molded by melting, which can be dissolved by certain acids, etc. Locke calls this the "nominal" essence of gold, that which allows us to attach the name "gold" to particular objects we encounter. But gold also has a real essence, that which is responsible for the observable qualities that to into the nominal essence. This real essence would be found in the corpuscles that make up gold objects, were we able to observe them. But we are not. The real essences of things are quite unknown to us and may always evade our grasp. This is a skeptical consequence of Locke's empiricism.
Having had a glimpse of Locke's theory of ideas, we may now have a brief look at his theory of knowledge. Here we find another skeptical consequence of Locke's empiricism. Locke concludes that human knowledge is somewhat limited, given the "narrow outlets" of the senses through which we obtain all our information about the world outside us.
Locke defined knowledge as the perception of agreement or disagreement of ideas with one another. In many cases we can determine merely by examining the ideas themselves whether they agree or disagree, and we have knowledge in those cases. But if we cannot detect from the ideas themselves whether they are connected or cannot be connected, we can only have opinions about whether they are, and we lack knowledge. So when we have knowledge will depend on which relations between ideas are open to discovery by the human mind.
Locke counted exactly four types of agreement or disagreement (Book II, Chapter 1).
The final type of agreement is quite different in kind from the first three. Each of them involved a comparison of ideas existing in our own mind. Locke makes the stronger claim that we have knowledge of real existence when an idea correponds to a really existing thing. But the existing thing is not an idea, so this seems to violate the definition of knowledge as perception of agreement or disagreement of ideas, at least taken in the sense of the immediate objects of thought. But if we were to think of knowledge as agreement of internal ideas with ideas construed as qualities of objects, then there is the question of how we could tell that there is such agreement, when the only thing before the mind is the internal ideas. Here we have the fatal seed of skepticism which was noticed by Berkeley, exploited by Hume, and denounced by Reid.
Beyond the classification of types of agreement and disagreement of ideas, there is a further distinction made in Chapter 2 regarding the "degrees" of knowledge. The degree of knowledge is the degree of security we have in the perception of agreement or disagreement of ideas. There are three degrees, listed in descending order of security.
Demonstrative knowledge is very secure, but it involves intermediate steps. Each step of a demonstration is known inuitively, but as the demonstration proceeds, the luster of the previous steps diminishes. Mathematical knowledge is demonstrative, as is knowledge of morality. Surprisingly, Locke held that we can even demonstrate properties of colors. (Here is a possible example of such a demonstration. One observes that shade A of red is deeper than shade B. Then one sees that shade B is deeper than shade C, and that shade C is deeper than shade D. Without having A and D both in view, one can conclude that shade A is deeper than shade D.) The only being whose real existence we know demonstratively is God.
Locke's "demonstrative" argument for the existence of God in Chapter 10 is quite unconvincing. He begins with the intuitive certainty that a real being cannot be produced by nothing. Given that something exists (for example, himself), it follows there is no time at which something did not exist. For whatever produced him must exist at a prior time, and if anything produced it, it would exist at a prior time, etc. At this point, Locke is guilty of a logical fallacy. He infers from the claim that at each time there is something that exists to the conclusion that there is something (a single thing, which he later identifies with God) that exists at each time.
Sensitive knowledge represents the lowest grade of security. "Going beyond bare probability and yet not reaching perfectly to either of the foregoing degrees of certainty, [it] passes under the name of knowledge" (Book IV, Chapter 3, Section 13). According to Locke, we have sensitive knowledge when an object exercises its power on our minds to produce simple ideas of its qualities and the complex idea of itself as a substance. When we cease to be influenced by the object, however, we have no more knowledge, but only probability. So besides ourselves and God, we only know the existence of sensible things that we are presently perceiving. Add to this our ignorance of the real essences of bodies, and our knowledge of them is very meager.
The following table briefly summarizes Locke's account of knowledge. "All ideas" indicate that it is possible to have this kind of knowledge regarding any two ideas. "No ideas" indicates that no two ideas can be perceived to agree or disagree in the way indicated. In the case of intuitive and demonstrative knowledge of real existence, the only objects of knowledge are listed. In all other cases, a single example is given.
| Identity | Relations | Co-existence | Real Existence | |
|---|---|---|---|---|
| Intuitive | All ideas | 3 > 2 | Whatever has a figure is extended | Myself |
| Demonstrative | No ideas | Pythagorean theorem | ? | God |
| Sensitive | No ideas | No ideas | ? | I am facing a tree |
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