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The Scientific Revolution and Enlightenment

John Locke’s Theory of Knowledge
(An Essay Concerning Human Understanding)

by Caspar Hewett
John Locke’s epistemology by Casper Hewitt

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John Locke Published in 1690, An Essay Concerning Human Understanding is the masterwork of the great philosopher of freedom John Locke. Nearly twenty years in preparation Locke began working on The Essay in 1670 following a series of philosophical discussion during which he and his friends decided that “it was necessary to examine our own abilities, and see what objects our understandings were, or were not, fitted to deal with.” The Essay is an attempt to establish what it is and isn’t possible for us to know and understand. “My purpose” Locke says, is “to enquire into the origin, certainty, and extent of human knowledge; together, with the grounds and degrees of belief, opinion, and assent.” The aim thus is not to achieve certainty, but to understand how much weight we can assign to different types of knowledge.

The Essay is divided into four books, the first three laying the foundation for the arguments set out in Book IV. Central to Locke’s argument throughout the Essay is the idea that when we are born the mind is like a blank piece of paper. He says:

    Let us then suppose the mind to be, as we say, white paper void of all characters, without any ideas; how comes it to be furnished? Whence comes it by that vast store, which the busy and boundless fancy of man has painted on it, with an almost endless variety? Whence has it all the materials of reason and knowledge? To this I answer, in one word, from experience: in that, all our knowledge is founded; and from that it ultimately derives itself.

What Locke is talking about here is the content of the mind, not its abilities. It is important to highlight this as the notion of the mind as white paper (or as a blank slate to use another popular metaphor) is one that is still contentious today and different people mean different things by it. Locke clearly believes that we are born with a variety of faculties that enable us to receive and process information (the senses, memory, our ability to use language, explored in some detail in Book III of the Essay) and to manipulate it once we have it, but what we don’t have is innate knowledge or ideas.

Book I of the Essay, Of Innate Notions is dedicated to refuting the hypothesis that we are born with imprinted or innate ideas and knowledge, something that puts him at odds with the thought of Descartes. But it is not just Descartes that he is refuting here. At the time it was widely thought that certain ideas and principles were imprinted on human beings from birth and that these were essential to the stability of religion and morality and I think this is one reason why Locke spends so much time debunking the notion of innateness. But there is much more to it than that. Locke believed deeply in humanity. He was not a secular thinker, in fact he was a devout believer in God, but he thought that the God-given faculties we possess, especially the ability to reason, gave us a unique place in nature which we should take full advantage of. Locke was a political animal, intimately involved in the changes taking place in England at the time, and a great believer in individual freedom. His was a political project and his interest in the mind had a practical purpose behind it – he wanted to transform society and organise it in a rational way. His rejection of innate ideas was intimately linked to this project for it is all too easy to claim all sorts of principles as innate in order to maintain the status quo, meaning that people “might be more easily governed by, and made useful to some sort of men, who has the skill and office to principle and guide them. Nor is it a small power, it gives one man over another, to have the authority to be the dictator of principles, and teacher of unquestionable truths; and to make a man swallow that for an innate principle, which may serve his purpose, who teacheth them.”

Let’s examine his argument. Consider for example the simple notion that it is not possible for something to both exist and not exist. Locke argues that if such a proposition were innate then every person in every period of history would know and understand this, but this is clearly not the case. If such truths were ‘imprinted’ on us all then we would expect that “children and idiots” would not only be fully aware of them, but also be able to articulate them. For Locke it makes no sense to imagine both that ideas or knowledge are innate and that we do not know them, thus in his own words: “It seems to me a near contradiction to say that there are truths imprinted on the soul, which it perceives or understands not; imprinting if it signify anything, being nothing else but the making certain truths to be perceived.” He goes on to take up the suggestion that innate propositions are only perceived under certain circumstances. The crux of his argument is that once we start to think in this way it becomes unclear what is meant by innate ideas at all – if we are not all aware of them nor able to perceive them can they really be described as innate? Accepting such a view would make it impossible to distinguish between innate ideas and new ideas that we discover.

He also takes up at some length the claim that innate propositions are discovered when people come to use reason. For Locke it makes no sense to describe a truth that is discovered through the use of reason as innate and he constructs a careful argument to back this up, investigating and refuting different interpretations of the claim. I do not have space here to go into too much detail here, but Locke goes on to reject the claim that there are innate practical moral principles or that we are born with innate ideas of God, identity or impossibility.

Book II of the Essay, Of Ideas, lays out how human beings acquire knowledge, beginning by making a clear distinction between different types of ideas. There are simple ideas which we construct directly from our experience and complex ideas which are formed by putting simple (and complex) ideas together. Locke divides complex ideas into three types which he describes as ideas of modes, substances and relations. Modes are “dependences on, or affectations of substances” and relations. Thus they are things that depend on us for their existence, including things as diverse as the ideas of gratitude, rectangle, parent, murder, religion and politics. Substances are things in the material world that exist independently, including what we would generally describe as substances such as lead and water, but also including beings such as God, humans, animals and plants and collective ideas of several substances such as an army of men or flock of sheep. Relations are ideas that consist “in the consideration and comparing one idea with another.”

Locke proposes that the mind puts ideas together in three different ways. The first is to combine simple ideas to form complex ones. The second is to bring two or more ideas together and form a view of them in relation to each other. The third is to generate general ideas by abstracting from specific examples. Thus we ignore the specific circumstances in which we gain a particular piece of knowledge, which would limit its applicability, and generalise so that we have some rule or idea that applies in circumstances beyond our direct experience. This interpolation and abstraction is important in a number of areas (morality for example) but is of course essential to science, and Locke’s familiarity with the mechanical philosophy provided part of the reason for emphasising this way in which we generate ideas. He goes on to discuss how sensation and reflection give rise to a number of kinds of ideas, including moral relations and ideas of space, time, numbers, solidity, identity and power.

By far the longest chapter in Book II is a discussion of power and this is particularly interesting in that it provides an opportunity to explore the notions of free will and human agency, which lie at the heart of Locke’s political project. Here we are not talking about power in the sense it is used in physics (the rate at which energy is used) nor about the power one person exerts over another, but rather in a much more general sense of an ability to make a change (active power) or receive a change (passive power). For example “fire has a power to melt gold … and gold has a power to be melted … the Sun has power to blanch wax, and wax has a power to be blanched by the Sun.” Thus “the power we consider, is in reference to the change in perceivable ideas.”

Locke’s primary interest in power is, unsurprisingly, not related to substances in general, but is in the abilities of human beings, in particular the powers or faculties of the mind such as liberty, will and desire. He defines liberty as “a power to act or not to act, according as the mind directs” whereas the will is a “power to direct the operative faculties to motion or rest in particular instances,” and argues that desire is an uneasiness “fixed on some absent good, either negative, as indolency to one in pain; or positive, as enjoyment of pleasure.” He is careful to distinguish between these powers and the person (the agent) who possesses them, for these faculties are not “real beings in the soul” that can perform actions – only the person acts. In a similar vein he argues that one power cannot operate on another, “it is the mind that operates, and exerts these powers; it is the man that does the action, it is the agent that has the power, or is able to do.”

Thus for Locke the idea of free will is nonsensical – a person can be free “to think, or not to think; to move, or not to move, according to the preference or direction of his own mind,” but the will cannot, for it is simply one of the faculties of a person – the will does not think, nor can it choose a course of action, thus how can it be free? In order to emphasise the distinct nature of the powers discussed he points out that “there may be thought, there may be will, there may be volition, where there is no liberty.” For example a man falling into water from a height “has not herein liberty, is not a free agent” since, although he would prefer not to fall he is not in a position to act on that preference. Similarly a man hitting a friend due to a convulsive movement of his arm would not be considered by anyone to have liberty in this as it is out of his control – he has no choice in the action.

Locke’s discussion of identity is also interesting in that it explores what we mean when we think of something retaining a particular identity. If we are dealing with an inanimate object this is quite straightforward, we simply have to ask whether it consists of the same matter, but if we are considering a living being things are not so straightforward: “a colt grown up to a horse … is all the while the same … though there may be a manifest change of the parts.” Here identity is associated with some continuity of life of the being in question rather than it consisting of the same matter. When it comes to humanity the question of identity becomes further complicated and Locke makes an important distinction between a human being (‘man’) and a ‘person’. The identity of a human being is the same as that of any other animal, defined by “participation of the same continued life,” but a person is “a thinking intelligent being, that has reason, and reflection, and can consider itself as itself, the same thinking thing in different times and places.”

Book III of the Essay, Of Words, is central to Locke’s epistemology or theory of knowledge. He explores the intimate connection between the names we give to things and ideas and, following the arguments detailed in Book II, links language and ideas directly, claiming that most words “are names of ideas in the mind.” He does deal with other types of word, such as particles that “signify the connexion that the mind gives to ideas, or propositions, one with the other” but his focus is on words that represent ideas in the mind. Thus most words can be classified according to the same categories as ideas were in Book II; words for substances, modes and relations.

He emphasises that when we use words they always represent the ideas the person speaking has in his or her head, which are not necessarily the same as the ideas associated with those words in the mind of the person listening. However, language is such that people generally assume they mean the same thing when they use a particular word and, further, “often suppose their words to stand also for the reality of things.” This leads him to explore different types of words, how we understand them, and how we use them to increase knowledge. He points out that most words are general terms arguing that if this weren’t the case language wouldn’t be much use for improving knowledge, for while knowledge is “founded in particular things” it “enlarges itself by general views.” He sees words as becoming general “by being made the signs of general ideas” and it is here that the intimate connection between words and ideas is key.

Locke claims that it is not possible to define the names of simple ideas, only complex ones, since simple ideas are rooted in the things that we sense and can only be named by reference to the things themselves: “Simple ideas … are only to be got by … impressions, objects themselves make on our minds.” He cites the problem of trying to define the meaning of the word light to a blind man as an example. Without the sense of sight it is not possible to understand any definition put forward in the way a sighted person can. Complex ideas, on the other hand, can be defined in terms of simple ideas, provided we are equipped with all the appropriate senses (e.g. sight) for understanding the simple ideas used. For example a rainbow can be defined in terms of its shape, the colours it consists of and the order they appear in.

Pointing to the non-universal nature of words and language, Locke points out that words in one language do not always have an equivalent in another “which plainly shows, that those of one country, by their custom and manner of life, have found occasion to make several complex ideas, and give names to them, which others never collected into specific ideas.”

Locke also discusses the essence of a sort or species of idea, by which he means “that abstract idea to which the name is annexed; so that everything contained in that idea, is essential to that sort.” He makes a distinction between the nominal and real essence of a sort. The nominal essence is the complex idea a word stands for, while the real essence is the true properties or constitution of the thing we describe by the word, some of which we may know, but many of which we usually don’t. This distinction is extremely important to Locke’s overall thesis since the aim of the Essay is to examine what we can and cannot know. For Locke the real essence of something is not something we can ever know, as there will always be some properties, or some behaviour that we are unaware of. Nominal essences on the other hand will vary from person to person. For example the “yellow shining colour, makes gold to children; others add weight, malleableness, and fusibility; and others yet other qualities …” However, we have to be very careful when we talk of real essences. For one thing we only suppose their being, without knowing what they are, but also the real essence of a substance such as gold “ relates to a sort” and thus is related to our abstractions and the words we assign to them; “our distinguishing substances into species by names is not at all founded on their real essences.” Inevitably the way in which we group substances into sorts or species is based on “their nominal, and not by their real essences … they are made by the mind.”

This whole account of essences, and indeed the deliberate use of the word essence, represents an important break from the essentialism of the Aristotelian tradition that Locke was taught in his youth. Aristotle believed that there are natural kinds, the essences of which can be organised into a single hierarchical system of classification which corresponds to the way nature is structured. Locke rejected this claim entirely. Rather than a unique classification open to discovery by the scientist Locke thought it useful to classify things in lots of different ways depending on what one wanted to do. This is quite a profound difference. It represents an important break with the thinking of the past and in this he was clearly influenced by natural philosophers such as his old friend and mentor Robert Boyle. Part of the reason for discussing words in Book III of the Essay is precisely to break down the idea of fixed boundaries between species or sorts of ideas. He says “these essences of the species of mixed modes, are not only made by the mind, but made very arbitrarily, made without patterns, or reference to any real existence.” In this he prefigures Charles Darwin, who needed to dispense with the concept of fixed species of animals in order to establish the theory of evolution by natural selection, by nearly 170 years!

It might seem from this discussion that Locke believed that words never retain a common meaning when they are used by one person speaking to another, but this is not the case. Locke, the master of common sense, was well aware that words must sometimes signify the same meaning to different people for otherwise there would be no communication and language would be completely useless. However, the more complex the idea signified by the word, the more likelihood that the word represents a different idea in the mind of each person who hears or reads it. For the most part Locke sees language as a tool for carrying out the pragmatic communication necessary in everyday life. Ordinary people are the creators of language: “Merchants and lovers, cooks and tailors, have words wherewithal to dispatch their ordinary affairs; and so, I think, might philosophers and disputants too, if they had a mind to be clearly understood.”

Book IV of the Essay, Of Knowledge in General, brings to bear the arguments in the previous books on Locke’s central question of what we can and cannot know. His approach is to deal with what knowledge is, how we reach it, what the different types of knowledge are and how certain we can be of any knowledge we gain. He defines knowledge in terms of whether or not one idea in our mind agrees with another (or others), thus it is “the connexion and agreement, or disagreement and repugnancy of any of our ideas.” This is significantly different from Descartes’ account of knowledge which defines it as any ideas that are clear and distinct. Here we can see why Locke is at such pains to make it clear what he means by ideas and their signs (words) before defining knowledge and embarking on the central question of the Essay. He argues that “all that we know or can affirm concerning any of” our ideas

    is, that it is, or is not the same with some other, that it does, or does not always co-exist with some other idea in the same subject; that it has this or that relation to some other idea; or that it has a real existence without the mind

and that “wherever the mind perceives the agreement or disagreement of any ideas, there be certain knowledge.”

He defines four sorts of agreement or disagreement: identity, relation, co-existence (or necessary connexion) and real existence giving the examples:

    ‘blue is not yellow,’ is of identity. ‘Two triangles upon equal basis, between two parallels are equal,’ is of relation. ‘Iron is susceptible of magnetical impressions,’ is of co-existence, ‘GOD is,’ is of real existence.

He distinguishes between three types of knowledge, which have different degrees of certainty. The clearest and most certain is intuitive knowledge, the second most certain demonstrative knowledge and the third sensitive knowledge.

Intuitive knowledge is that where “the mind perceives the agreement or disagreement of two ideas immediately by themselves, without the intervention of any other.” For example ‘white is not black,’ ‘a circle is not a triangle,’ ‘three is greater than two.’

Demonstrative knowledge is that where the agreement or disagreement is not perceived immediately, but rather depends on reasoning – following a series of steps in the mind, each of which must have intuitive certainty, to discover the agreement or disagreement of ideas “by the intervention of other ideas.”

    Those intervening ideas … are called proofs, and where the agreement or disagreement is by this means plainly and clearly perceived, it is called demonstration, it being shown to the understanding, and the mind made see that it is so.

Because of all the steps involved in achieving this sort of knowledge it is seen as “more imperfect than intuitive knowledge.” This sort of proof is common in my discipline of mathematics, but Locke is arguing that this type of reasoning is valid in all areas of knowledge.

As an illustration I am going to show you a simple demonstrative proof of one of Locke’s examples: that if we add the three angles in a triangle together they are the same as two right angles. I will not use any mathematical symbols as I know this will put off at least two thirds of my readers, but will rather use a series of diagrams. The idea, remember, is that each step should have intuitive certainty in order to provide proof of the hypothesis through reasoning and I hope that the example I have chosen will carry you with it.

First we remind ourselves that a right angle is the angle we find in a square or rectangle, looking like a capital ‘L’, see [1] in the diagram below. I am only going to show you the proof for an acute angled triangle (one with no angles larger than a right angle), so let’s start with a general acute angled triangle as shown in [2]. If we take an identical triangle and turn it upside down as shown in [3], then bring the two triangles together as in [4] then we have the shape shown in [5] which we describe as a parallelogram. We can see that each of the three angles a, b, c in our original triangle appear twice in the parallelogram.

If we look at the top left corner of the parallelogram in [5] (labelled D in [6]), I can draw a line from there to the base of the parallelogram to make a right angle with the base as shown in [6]. Now what we have is a right angled triangle on the left and a four sided figure on the right that can be separated as in [7]. The triangle can be moved over to the right hand side of the diagram, where, because of the size of the angles it will fit exactly onto the other figure, making a rectangle, see [9] and [10].

Thus we have demonstrated, by means of diagrams that the three angles a, b, c in our original triangle, when doubled (two triangles in [3]) have angles adding up to four right angles (in the rectangle in [10]). Thus angles a, b and c add up to two right angles. There are other ways of proving this, but I quite like this diagrammatic proof by demonstration for its appeal to our intuitive feeling for shapes and how they fit together.

The last type of knowledge Locke discusses, sensitive knowledge, is the least certain as it is founded on objects that enter our minds directly through the senses. Locke is well aware of the doubts associated with trusting our senses but, ever the common-sense philosopher, argues strongly that it makes no sense to reject the input we receive from the outside world. We should accept that things in the external world have a real existence even if our knowledge of them will always be imperfect:

    The notice we have by our senses, of the existence of things without us, though it be not altogether so certain, as our intuitive knowledge, or the deductions of our reason … deserves the name of knowledge.

Continuing on this theme, Locke claims that it is not possible for us to discover the connection between what he describes as the primary and secondary qualities of a substance. The term primary qualities refers to the ‘real’ attributes of a substance, such as its size, shape and motion while the secondary qualities are those that we sense such as colour, taste or sound. The problem is that, while there is no doubt a connection between these different types of quality, nothing in the substance itself truly resembles its secondary qualities. It is simply that the physical attributes of the substance, its primary qualities, have “a power to produce those sensations in us.” Thus, while arguing that we should trust that our senses provide real, if imperfect, knowledge of the physical world (sensitive knowledge), he also severs the connection between simple ideas (in this case secondary qualities) and reality.

This leads on to a consideration of probability or likelihood of truth. We have to accept the lack of certainty associated with our understanding of the physical world because of our reliance on our senses, but this does not mean that we cannot make rational judgements about what we observe. Locke presents an account of probable reasoning which is very similar to the demonstrative reasoning that generates knowledge. However, not every step in probable reasoning has intuitive certainty, only a certain likelihood of truth. Thus when we judge an argument or proposition as true or false we cannot guarantee that our judgement is correct, only that it is more or less likely. Therefore there are degrees of such judgement ranging from near certainty to highly improbable. Locke’s discussion of probable reasoning in the Essay does deal with things that we can observe and experience, but his focus is on things beyond our senses including immaterial spirits such as angels, things too small to sense such as atoms and life on other planets, which we cannot sense because of their remoteness from us. However, I want to draw attention to the profound importance of his points about probable reasoning if we are to have a true appreciation of the strengths and limits of the scientific method.

This search for knowledge through probable reasoning is one way of thinking about what the sciences are all about –when we assess a theory or hypothesis we balance probabilities. What is more likely? Why? At every step of an argument we should be weighing up our level of certainty. In general, because we are rarely dealing with ‘intuitive certainty’, the more steps, the less certain we are of our conclusions. However, the more experiments and observation we can perform related to each step to confirm or refute our assumptions, the more certain we can be. This is very important to appreciate and unfortunately is not appreciated by a lot of scientists! It is also a huge problem for the sciences of humanity – human beings are so complex and so different from one another that it is surprisingly difficult to construct general arguments about humanity that hold up to this kind of scrutiny.

So, what can and can’t we know? Like Descartes, Locke argues that we can be certain of our own existence, this falling into his category of intuitive knowledge, and we have “a demonstrative knowledge of the existence of God.” Regarding “the real, actual existence … of anything else, we have no other but a sensitive knowledge.” However, there are areas of knowledge, such as mathematics and morality, which are capable of demonstration and thus a high level of certainty. This is because they are closed systems in which the rules are created in our minds – they do not depend on input from our senses. He uses two telling examples: ‘Where there is no property, there is no injustice,’ is certain

    for the idea of property, being a right to any thing, and the idea to which the name injustice is given, being the invasion or violation of that right; it is evident, that these ideas being thus established, and these names annexed to them, I can as certainly know this proposition to be true, as that a triangle has three angles equal to two right ones. Again, ‘no government allows absolute liberty’: the idea of government being the establishment of society upon certain rules or laws, which require conformity to them; and the idea of absolute liberty being for anyone to do whatever he pleases, I am as capable of being certain of truth in this proposition, as of any in mathematics.

However, it is difficult to establish certain truths in ethics because of the complexity of moral ideas and this where the discussion of language in Book III becomes most pertinent: Locke draws attention to two ‘inconveniences’ that are a consequence of this complexity. First, that the words we use, the ‘names’ assigned to moral ideas, are less precise than those of, say, mathematics, thus the idea carried in one mind by a certain word may differ from that in another mind. Secondly, that it is difficult for the mind to remember precisely all the relationships between different ideas and thus, especially when several complex moral ideas are involved, it can be very difficult to decide on the agreement or disagreement of ideas being compared (which, remember is Locke’s definition of how we come to knowledge). Morality does not have the advantage that mathematics has of being able to use diagrams (and precisely defined symbols) which allow you to review each stage of a demonstration with ease.

Following this train of thought, Locke moves on to the extent of our knowledge “in respect of universality,” arguing that only abstract general ideas can provide any sort of universal knowledge:

    If the ideas are abstract, whose agreement or disagreement we perceive, our knowledge is universal. For what is known of such general ideas, will be true of every particular thing, in whom that essence, i.e. that abstract idea is to be found: and what is once known of such ideas, will be perpetually, and for ever true. So that as to all general knowledge, we must search and find it only in our own minds, and ‘tis only the examining of our own ideas, that furnishes us with that.

Only truths belonging to abstract ideas are eternal “as the existence of things is to be known only from experience.” This further underlines Locke’s arguments concerning morality for “the truth and certainty of moral discourses abstracts from the lives of men, and the existence of those values in the world, whereof they treat.”

He also warns against confusing ideas with the words we assign to them as “the examining and judging ideas by themselves, their names being quite laid aside” is “the best and surest way to clear and distinct knowledge.”

In Chapter X Locke lays out how we can be sure of the existence of God. I will not go into the details of his argument here, but do think it of interest to pick out two key points that lie at the heart of his reasoning and which I think are philosophically flawed. The first is that it is inconceivable that there was ever a time when there was nothing – for this he appeals to our intuitive certainty that “bare nothing” could not possibly produce any real being. Thus there must be an eternal being “since what was not from eternity, had a beginning, and what had a beginning, must be produced by something else.” He goes on to reason that “the eternal source then of all being must also be the source and original of all power; and so this eternal being must be also the most powerful” and also must be a “knowing intelligent being,” as there is no other way that humans, who are knowing intelligent beings themselves, could have come into existence:

It being as impossible, that things devoid of knowledge, and operating blindly, and without any perception, should produce a knowing being, as it is impossible, that a triangle should make itself three angles bigger than two right ones. For it is as repugnant to the idea of senseless matter, that it should put into itself sense, perception and knowledge, as it is repugnant to the idea of a triangle, that it should put into itself greater angles than two right angles.

Like the earlier discussion of species this argument bears an interesting relationship to ’s theory of evolution by natural selection, which did not come along for another 170 years. provides us with an alternative: his theory explains how a blind process can generate sense, perception and intelligence. I discuss this at length elsewhere, but thought it worth drawing attention to while dealing with Locke’s ideas. However, as with any of the great thinkers, it is also worth remembering when Locke was writing and not take it out of context. Locke believed in questioning everything and in not accepting the authority either of the past or of the clergy. He wanted people to rely on their own judgement and reasoning which is precisely why he constructs an argument to justify believing in God, and that is interesting in its own right. I will return to this theme at the end of the chapter.

Having found the bounds of human knowledge and certainty Locke turns to the various degrees of probability or likelihood of the truth of an idea. This is the area of human knowledge where, in the absence of certainty, we have to apply our judgement. Here our minds have to take ideas to agree or disagree or take some proposition to be true or false “without perceiving a demonstrative evidence in the proofs.”

The highest degree of probability follows from what our own and other people’s “constant observation has found always to be after the same manner,” for example that fire burns. We cannot prove that fire burns in all circumstances, but our experience and what we know of the experience of other people gives us no reason to doubt that it will continue to do so in conditions we have yet to come across. These probabilities rise near to certainty and we generally don’t distinguish between them and certain knowledge. The second degree is when “my own experience, and the agreement of all others that mention it, a thing to be, for the most part, so: and the particular instance of it is attested by many undoubted witnesses.” This degree of probability, while less certain than the first degree, we tend to have confidence in, and will generally be willing to act on as if it were fact. The third degree, which is of course the weakest, is based on what Locke calls ‘fair testimony.’ This is when we are told that something, confirmed by witnesses, happened at a certain time and place and, having no contradiction or reason to disbelieve the account, we believe it.

Locke draws attention to the difficulties associated with probabilistic reasoning, particularly when something contradicts common experience, or when different witnesses or histories give a different account of events. However, we should always try as best we can to assess the likelihood of an account for ourselves and should not fall into the trap of discounting something which is counter to our own experience – this may simply reflect that our own experience is limited! This is good advice for any scientist as much of science seems at face value to contradict common sense (does the Earth appear flat or curved to you?) – it is only when we investigate further (experiment, observation) or look at the right scale that the properties or behaviour of an object are revealed.

In the closing chapters of the Essay Locke makes a number of points about reason, faith and judgement which stand today as useful guidelines for how we should approach knowledge. He urges us to trust our own judgement and to consider the probability of any proposition for ourselves. He makes the interesting point that repetition of a single testimony should give it no more weight than if it were only heard once. His point here was primarily aimed at the word of the ancients, and had a bearing on the general point about rejected authority and trusting oneself. It is also another point highly relevant to the modern era, especially in this age of instant messaging and the web, where a single testimony can be repeated a million times extremely rapidly without any verification of facts or truth. It is always worth distinguishing between a variety of sources confirming something and a number of sources repeating the same rumour!

Locke is explicitly against artificially formalised types of reasoning, attacking at length the use of syllogism, a highly formal type of argument favoured by Aristotle and his followers. Rather he makes the case for argument from judgement as the only sort of argument that brings true instruction and advances us in our way to knowledge. He describes it as “the using of proofs drawn from any of the foundations of knowledge, or probability.” Its validity arises from it relying solely on reason, not on respect for the reputation of some kind of authority, nor on accepting an argument simply because we do not know a better one.

Locke makes a point of refuting the idea that reason is opposed to faith, claiming that faith can never convince us of anything that contradicts our knowledge and arguing that, except in the case of divine revelation, we should always look first to our own reason. Thus anything worldly and open to our own deduction, observation, experiment or experience must always be a matter of reason. The only times where it is appropriate to resort to faith alone is in areas not open to our enquiry such as whether there is an afterlife or whether angels exist.

C J M Hewett, November 2006


Locke, John (2004) An Essay Concerning Human Understanding, Penguin Classics

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An Essay Concerning Human Understanding (1690)
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A Letter Concerning Toleration
Of the Conduct of the Understanding
Some Thoughts Concerning Education (1693)

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An Essay Concerning Human Understanding Two Treatises of Government & A Letter Concerning Toleration Second Treatise of Government Some Thoughts Concerning Education On Sovereignty by Thomas Hobbes, John Locke
The Enemies of Hope: A Critique of Contemporary Pessimism Locke on Government The Cambridge Companion to Locke Hobbes, Locke, and Confusion's Masterpiece Death of the Subject Explained
Man, Beast and Zombie The Blank Slate Evolutionary Psychology: A Critical Introduction Science and the Retreat from Reason Nature via Nurture

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