Locke on Knowledge

Locke has a scholastic conception of knowledge proper, just as Descartes does, but unlike Descartes, Locke thinks such knowledge is achievable in ethics and mathematics only, and thus, contra Descartes, not in metaphysics or natural science.

The Nature of Knowledge

Locke defines knowledge as a relation between our ideas:

Since the mind, in all its thoughts and reasonings, hath no other immediate object but its own ideas, which it alone does or can contemplate, it is evident that our knowledge is only conversant about them…Knowledge then seems to me to be nothing but the perception of the connexion of and agreement, or disagreement and repugnancy of any of our ideas. In this alone it consists. (IV.i.1-2)

There are three points of note here. First, that knowledge is a relation between ideas, not (at least in any immediate way) between a knower and the world. While this may be fine for stipulative knowledge (or even a priori knowledge generally), such as that a bachelor is an unmarried male, it does not seem to fit with existential knowledge of contingent matters, such as the color of one’s shirt, or that one had French toast for breakfast. These seems to be cases where one has knowledge of matters of fact or of particular individuals and their characteristics, rather than one’s ideas of them.

Second, Locke puts significant emphasis on the subject’s own perception of the agreement or disagreement of their ideas. Hence, for as subject to count as a knower, she must herself perceive, or be in a position to perceive, the requisite agreement or disagreement between her ideas. Locke’s political philosophy emphasizes the authority and sovereignty of the individual with respect to matters of right, and this same emphasis can be seen in the importance of one’s subjective perception for knowledge. For Locke, one cannot know things whose justification or evidence is taken merely on the authority of another (whether that be friend, parent, church, or state). One’s ultimate status as a knower stands or falls with the relation to one’s own ideas and not anyone else’s.

Third, Locke’s conception of knowledge, properly so called, ties knowledge to certainty. If one knows that p then one is certain that p. Here the notion of certainty is not that of a mere subjective state of conviction, it is rather factive — being certain that p entails that p is true. This means that there is no knowledge, properly so called, in cases where there is merely the probability of p’s being true. This high standard for knowledge is something Locke shares with both Descartes and with the Aristotelian scholastic tradition.

A further point, not made explicitly in the passage above, is that the connections that Locke has in mind concerning our ideas are necessary connections. He makes this point more clearly in his discussion of the limitations of our knowledge concerning the natural world.

I fear the weakness of human understanding is scarce able to substitute another [than the corpuscularian hypothesis], which will afford us a fuller and clearer discovery of the necessary connexion and coexistence of the powers which are to be observed united in several sorts of [bodies]…I doubt whether, with those faculties we have, we shall ever be able to carry our general knowledge (I say not particular experience) in this part much further. (IV.iii.16)

Knowledge is concerned with the necessary connections between ideas, and ultimately, between qualities in the bodies causing those ideas in us. But, Locke argues, we can have no knowledge of these necessary connections, and can only frame hypotheses concerning their possible nature (Locke’s favorite being the corpuscularian hypothesis).

Varieties of Agreement/Disagreement

If knowledge is agreement or disagreement between ideas, one might ask whether all such relationships are of the same form. Locke thinks that there are in fact four general categories into which such agreement/disagreement might fall.

  1. Identity, or diversity

    • in virtue of perceiving an idea one is aware that each idea is identical with itself and different from others (IV.i.4)
  2. Relation

    • perception of relations between ideas, even if only juxtaposing them in a single thought (IV.i.5)
  3. Co-existence, or necessary connection

    • ideas of characteristics being necessarily connected in a single entity, such as the color and texture of a particular piece of gold (IV.i.6)
  4. Real existence

    • perception of the actual existence of something. We have intuitive knowledge of our own existence, and know the existence of mind-independent objects via sensation (IV.xi.1)

‘Blue is not yellow’ is of identity; ‘Two triangles on equal bases between two parallells are equal’ is of relation; ‘Iron is magnetizable’ is of co-existence; and ‘God exists’ is of real existence. Though identity and co-existence are themselves relations, they are such special kinds of agreement or disagreement amongst ideas that they deserve to be brought in separately, not under relation in general. (IV.i.7)

‘Degrees’ of Knowledge

Locke also distinguishes between types or ‘degrees’ of knowledge. Another way of putting this is to say that though, for Locke, all knowledge is marked by a particular kind of perception of agreement or incompatibility between ideas, there are nevertheless different routes to such perception, different ways of the agreement’s being evident to one (IV.ii.14).

The most certain is ‘intuitive’ knowledge, which is phenomenologically immediate, and non-inferential.

Sometimes our mind perceives the agreement or disagreement of two ideas immediately—by themselves, without the intervention of any other: and this I think we may call intuitive knowledge…[in such knowledge] the mind perceives that white is not black, that a circle is not a triangle, that three are more than two and equal to one plus two. Such kinds of truths the mind perceives at the first sight of the ideas together, by bare intuition; without the intervention of any other idea: and this kind of knowledge is the clearest and most certain that human frailty is capable of. (IV.ii.1)

The second kind of knowledge is ‘demonstrative’ knowledge, which is mediated by inferences involving intermediate ideas.

The next degree of knowledge occurs when the mind perceives the agreement or disagreement of any ideas, but not immediately…The reason why the mind cannot always perceive presently the agreement or disagreement of two ideas, is, because those ideas, concerning whose agreement or disagreement the inquiry is made, cannot by the mind be so put together as to show it. In this case then, when the mind cannot so bring its ideas together as by their immediate comparison, and as it were juxtaposition or application to one another, to perceive their agreement or disagreement, it is fain, by the intervention of other ideas…to discover the agreement or disagreement which it searches; and this is what we call reasoning. (IV.ii.2)

Proofs are understood by Locke to be the means by which we reason to our intended conclusion, and the successful knowledge-yielding proof is a ‘demonstration.’

Demonstrative knowledge, though certain, is nevertheless less obvious or convincing than intuitive knowledge, partly due to limitations in our ability to remember and hold before us all the relevant steps of a proof.

to make anything a demonstration, it is necessary to perceive the immediate agreement of the intervening ideas, whereby the agreement or disagreement of the two ideas under examination (whereof the one is always the first, and the other the last in the account) is found. This intuitive perception of the agreement or disagreement of the intermediate ideas, in each step and progression of the demonstration, must also be carried exactly in the mind, and a man must be sure that no part is left out: which, because in long deductions, and the use of many proofs, the memory does not always so readily and exactly retain; therefore it comes to pass, that this is more imperfect than intuitive knowledge, and men embrace often falsehood for demonstrations. (IV.ii.7)

Anything that falls short of intuition or demonstration is not itself knowledge, properly so called. It is instead either ‘Belief’ or ‘Opinion’ (IV.ii.14). Locke makes an exception, however, for what he calls ‘sensitive knowledge’, which is less certain than either intuitive or demonstrative knowledge but which is something other than a claim whose truth is merely probable.

There is, indeed, another perception of the mind, employed about the particular existence of finite beings without us, which, going beyond bare probability, and yet not reaching perfectly to either of the foregoing degrees of certainty, passes under the name of knowledge. There can be nothing more certain than that the idea we receive from an external object is in our minds: this is intuitive knowledge. (IV.ii.14)

The obvious question, with respect to sensitive knowledge, is whether it is correct to think of it as genuine knowledge of the existence of mind-independent objects. What about the kinds of skeptical doubts raised by Descartes in the the initial stages of the Meditations? Locke thinks that we have intuitive certainty of the existence of our ideas of external objects (IV.ii.14), the question is whether we know of the existence of those objects themselves. Locke raises and answers the questions as follows:

whether there be anything more than barely that idea in our minds; whether we can thence certainly infer the existence of anything without us, which corresponds to that idea, is that whereof some men think there may be a question made; because men may have such ideas in their minds, when no such thing exists, no such object affects their senses. But yet here I think we are provided with an evidence that puts us past doubting. For I ask any one, (1) Whether he be not invincibly conscious to himself of a different perception, when he looks on the sun by day, and thinks on it by night; when he actually tastes wormwood, or smells a rose, or only thinks on that savour or odour? (2) We as plainly find the difference there is between any idea revived in our minds by our own memory, and actually coming into our minds by our senses, as we do between any two distinct ideas. If any one say, a dream may do the same thing, and all these ideas may be produced in us without any external objects; he may please to dream that I make him this answer:— (3) 1. That it is no great matter, whether I remove his scruple or no: where all is but dream, reasoning and arguments are of no use, truth and knowledge nothing. (4) 2. That I believe he will allow a very manifest difference between dreaming of being in the fire, and being actually in it. (IV.ii.14)

I’ve marked out four distinct issues Locke raises concerning skeptical doubt of the existence of an external world. We’ll take them in turn.

First, Locke points out that there is a distinctive phenomenology to our perceptions that cannot be replicated in thought. This is clearly correct, but doesn’t tell us anything about the veracity of our perceptions, which is what was in question.

Second, Locke points out that there is a clear difference, again, presumably phenomenologically, between memory and an occurrent perception. Locke is surely correct that we do not typically confuse our memories with occurrent experience, but it is not obvious that a memory and an occurrent experience could never be phenomenally indistinguishable. In abnormal circumstances one might have especially vivid memories that, at least in some respects, might closely if not exactly resemble the original experience. And once again, even if memory is always distinguishable from occurrent perception, this does not show that occurrent perception is giving any sort of cognitive access to things without the mind.

Third, Locke simply denies that anything like reasoning or argument would matter if our epistemic status were on par with dreaming. This is not so much an argument against the possibility that we are radically deceived, as we might be if dreaming, as it is the claim that if we are so deceived, then arguments don’t matter.

Finally, in the fourth point Locke quips that there is something very obviously different between being in a fire and dreaming or hallucinating being in a fire. This is certainly correct. One is painful, the other isn’t (or not as much). There are also clear differences in practical import. Presumably, actually being in the middle of a fire would lead to all sorts of (avoidance) behaviors that merely dreaming or hallucinating being in the fire might not. Has Locke made a significant point here? Not obviously. Descartes is quite clear that his concern is with slight or ‘metaphysical’ doubt that may have no practical import at all (7:22).

What comes through most clearly in these arguments is Locke’s disdain for the question. He has not, it seems, given any satisfactory answer to the question as to whether an external world really exists, but he doesn’t seem to think it a serious or pressing question.

Note though, that sensitive knowledge is not very extensive. Locke considers it to extend no further than our actual occurrent sensory experience (IV.iii.5). Hence, neither remembered past sensory experience, nor possible future sensory experience is available for underwriting knowledge of the external world. Thus, whatever knowledge we may have of particular occurrent existent things via sense experience, it will be limited to the present moment of consciousness.

The Prospects for Scientific Knowledge

Our contemporary notion of scientific knowledge only came into existence in the 19th century. The term ‘scientist’ was coined in 1834 by William Whewell. Before then, and going back to Aristotle, the dominant conception of theoretical knowledge, known as ‘Scientia’ (Greek: ‘Epistêmê'), concerned any organized body of knowledge which demonstrated necessary connections between known truths. Scientific knowledge could be stated in syllogistic argument, reasoning from more fundamental truths, to the truths (logically) derivable from them.

We’ll look both at the notion of a scientia as a system of logical demonstrations and Locke’s pessimism that natural science could ever satisfy such stringent requirements. One of Locke’s great contributions to epistemology was carving out a way between the traditional and highly demanding notion of scientific knowledge and a resolute skepticism. Locke attempted to articulate a notion of human knowledge as limited and finite, but as more than mere skepticism. For more extensive discussion see Kochiras (2013).

The Ideal of Demonstrative Knowledge

Aristotle puts the notion of scientific knowledge this way:

We suppose ourselves to possess unqualified scientific knowledge of a thing, as opposed to knowing it in the accidental way in which the sophist knows, when we think that we know the cause on which the fact depends, as the cause of that fact and of no other, and, further, that the fact could not be other than it is….The proper object of unqualified scientific knowledge is something which cannot be other than it is. [@aristotle1993, I.2]

So, according to Aristotle (and here the scholastics follow him), scientific knowledge is knowledge of asymmetric dependence relations that hold necessarily (in some sense of ‘necessary’). Typically, the sense of necessity at issue was thought to be captured by the logical necessity with which a conclusion follows from its premises in a a logical syllogism. Syllogisms are logical arguments of a particular form in which a predicate is either affirmed or denied of a subject via some intermediate proposition. For example,

  1. Socrates is human
  2. All humans are mortal
  3. ∴ Socrates is mortal

Here the predicate is predicated of Socrates via the intermediate proposition . The chaining together of predicates make evident how Socrates’s mortality is grounded in his humanity—i.e. in his being a human being. So we explain a particular feature of Socrates by saying that he is a being of a particular kind—a human being.

Now, with demonstrative knowledge we have an understanding of the conclusion as being necessitated by the relevant subject matter of the premises. However, it might seem that, in the case of Socrates’s mortality above (the actual fact of his mortality, rather than the logical premise), there is no such necessitation relation present. However, this is not how the Aristotelian conceives of things. The above syllogism attempts to map logically the metaphysically relevant ground or explanation—in this case Socrates’s humanity. In Aristotelian terms, it is the nature of being human to be mortal, and it is the fact that Socrates is human—a member of the species human—that determines, grounds, and thus explains, his being mortal, and it is this fact that is made explicit in the above syllogism. Another way of putting this is that it is a fact about Socrates’s essence—his being human—that explains why he is mortal.

Essence’, for Aristotle, picked out that feature or features of a thing that explain why it is the thing that it is. Essences thus determine (i) kind membership for any individual (e.g. that Socrates is of the kind ) and (ii) explain why the particular individual has the manifest properties that it does. It is this sort of explanation that is made explicit in deductive syllogisms like the one concerning Socrates above.

The ultimate aim of a science is to provide knowledge from the reason, cause, or ground of a thing, so as to explain why it is the way that it is. This requires appeal to what Aristotle called a “simple real definition” as opposed to a “compound nominal definition”. Compound definitions denote pseudo-individuals or kinds in the sense that they name items of different categories, most standardly substances and accidents. Hence “musician” is a compound definition denoting a kind of thing—a musical person—which does not have a genuine essence of its own. Musicians are not the kind of thing which are taken to have a common essence which explains why all things of that kind are the way they are (in contrast, say, to “human” or “animal”). No individual goes out of existence by ceasing to be musical (not even a musician, for there are, unfortunately, tone-deaf musicians) while a (human) musician clearly goes out of existence by ceasing to be human.

Nominal definitions, in contrast, do allow for distinguishing genuine (as opposed to pseudo) individuals, but do so in a way that fails either to identify the principle attribute of the thing or effect a proper division of its genus. To use an example from Locke (ECHU III.x.17), we consider rational animal a better definition of a human being than two legged animal with broad nails because the latter, though (let us say) extensionally correct, fails either to capture the principle characteristic of being human (rationality) or effect a proper division of its genus (animal).1

So, according to the Aristotelian conception of science, scientific inquiry aims at providing knowledge of why something is the way it is, which in turn requires the articulation of a real definition. Real definitions characterize the essences of individuals in a given domain of inquiry. The individuals in this domain are thus fundamental (in the sense of being explanatorily primary). Aristotle’s name for such a fundamental individual is “substance” (ousia). Given real definitions of substances an inquirer could use general principles, in which the real definitions figure, to provide a priori deductive demonstrations of the non-accidental (i.e. necessary but non-essential) features of any possible object in the relevant domain. Hence the possibility of scientific knowledge depends on the possibility of providing real definitions of things—substances—which thereby constitute the most explanatorily basic entities in the relevant domain of inquiry.

Locke on the Extent of Human Knowledge

Locke distinguishes our epistemic attitudes (what he calls ‘Assent’) into three categories—Knowledge, Belief, and Opinion. As we have seen, there are three sorts of knowledge—intuitive, demonstrative, and sensitive. When it comes to existing things Locke thinks our knowledge is very limited, concerning only what is present to one’s current consciousness.

Our knowledge of real existing things is thus quite restricted. What about scientific knowledge (scientia) of the kind articulated by Aristotle? Possession of such knowledge isn’t directly of existing things, since we might have knowledge of the essences of things that don’t actually exist. A good example of this would be the objects of Euclidean geometry. The Euclidean definitions of the plane figures articulate the conditions for the existence of those figures (e.g. for anything to be a triangle, its angles must sum to 180˚), whether or not they actually do exist. Unsurprisingly then, Locke does think that we have scientific knowledge of geometry (IV.ii.10).

Locke also thinks that we have scientific knowledge of morality.

Where there is no Property, there is no Injustice, is a Proposition as certain as any Demonstration in Euclid: 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 on the strength of these two Ideas and the 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. (IV.iii.18).

So morality is characterized by determinate relationships between ideas, that we can know demonstratively, and thus is a body of scientific knowledge as sure any bit of mathematics.

Can we have scientific knowledge of the natural world? Put another way, is natural philosophy a form of scientia? Answering this question positively would be to say that we could have knowledge of the essences of material substances, and thus of the necessary connections between the essences and the other features of the substances whose essences they are ( Locke’s radical position (here in stark contrast to Descartes’s) is that we cannot have such knowledge, and thus that natural philosophy is not a form of scientia. Locke gives two reasons for thinking this.

The first argument he provides is a kind of argument from access. The worry he raises is that our access to mind-independent objects is largely via knowledge of their secondary qualities.

The ideas that our complex ones of substances are made up of, and about which our knowledge concerning substances is most employed, are those of their secondary qualities; which depending all (as has been shown) upon the primary qualities of their minute and insensible parts; or, if not upon them, upon something yet more remote from our comprehension; it is impossible we should know which have a necessary union or inconsistency one with another. For, not knowing the root they spring from, not knowing what size, figure, and texture of parts they are, on which depend, and from which result those qualities which make our complex idea of [for example] 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 co-exist with that complex idea we have of it, or else are inconsistent with it. (IV.iii.11)

Our knowledge of primary qualities is greatly limited, even obscure, Locke thinks. We may have knowledge that all the parts of a body must have the determinable properties of shape, size, motion, and solidity, but by and large we do not know the determinates of these properties, so we do not know which determinate qualities go with which others, nor how they combine to produce the wide variety of secondary qualities characteristic of the macroscopic world.

This connection between the primary and secondary qualities is something that Locke seems to think is totally inscrutable to us (IV.iii.12-13). According to Locke there is simply “no discoverable connection” between the determinate primary qualities of a body and its secondary qualities.

We are so far from knowing what figure, size, or motion of parts produce a yellow colour, a sweet taste, or a sharp sound, that we can by no means conceive how any size, figure, or motion of any particles, can possibly produce in us the idea of any colour, taste, or sound whatsoever: there is no conceivable connexion between the one and the other. (IV.iii.13)

Locke then sums up his argument as follows:

In vain, therefore, shall we endeavour to discover by our ideas (the only true way of certain and universal knowledge) what other ideas are to be found constantly joined with that of our complex idea of any substance: since we (1) neither know the real constitution of the minute parts on which their qualities do depend; nor, (2) did we know them, could we discover any necessary connexion between them and any of the secondary qualities: which is necessary to be done before we can certainly know their necessary co-existence. So, that, let our complex idea of any species of substances be what it will, we can hardly, from the simple ideas contained in it, certainly determine the necessary co-existence of any other quality whatsoever. Our knowledge in all these inquiries reaches very little further than our experience. (IV.iii.14)

Thus we cannot have scientific knowledge in natural philosophy because (1) we cannot know the real essences of the minute bodies composing the observable world, and (2) even if we could know them we could not discern the necessary connections that exist between those bodies and their observable secondary qualities. We might summarize these points by saying that our grasp of the determinate primary qualities of things is at best obscure, while the connection between these determinate primary qualities and either determinate or determinable secondary qualities is completely inscrutable.

Locke does acknowledge that we have very limited knowledge of the necessary connections that exist between some of the primary qualities of things.

Some few of the primary Qualities have a necessary dependence, and visible connexion one with another, as Figure necessarily presupposes Extension, receiving or communicating Motion by impulse, supposes Solidity. (IV.iii.14)

But knowledge of such connections is very limited, based partly on the fact that our knowledge of determinate primary qualities of corpuscles is limited by their unobservability (in fact if not in principle, see [@downing1992]), and in part by the possibility that the physical world is causally interconnected in ways such that our ignorance of extremely remote objects (e.g. astronomically remote planets, etc.) would entail our ignorance of aspects of bodies closer to us (cf.

Is Locke a Skeptic?

If Locke denies that we can have proper scientific knowledge of the natural world, does this make him a skeptic?

Recall that Locke argues for a kind of knowledge, inferior to intuitive and demonstrative knowledge, but nevertheless worthy of the name—viz., ‘sensitive’ knowledge. Sense experience gives us knowledge of the existence of currently perceived objects via their perceived properties. This does not give us any knowledge of the real essences of things but only what Locke calls their ‘nominal’ essence—that set of characteristics which we use to sort things into kinds (e.g. , ). Our knowledge of the coexistence of such properties as to make it count as a thing of a certain kind (e.g. as a dog or a piece of gold) is limited to probable rather than certain connections.

For all the Qualities that are co-existent in any Subject, without this dependence and evident connexion of their Ideas one with another, we cannot know certainly any two to co-exist any farther, than Experience, by our Senses, informs us. Thus though we see the yellow Colour, and upon trial find the Weight, Malleableness, Fusibility, and Fixedness, that are united in a piece of Gold; yet because no one of these Ideas has any evident dependence, or necessary connexion with the other, we cannot certainly know, that where any four of these are, the fifth will be there also, how highly probable soever it may be. (IV.iii.14)

For sensitive knowledge to count as knowledge proper of the natural world, it must satisfy two conditions. First, the complex idea which we generate via an act of combination on a set of simple ideas must concern simple ideas which we have actually encountered as coexisting in nature.

Herein therefore is founded the reality of our Knowledge concerning Substances, that all our complex Ideas of them must be such, and such only, as are made up of such simple ones, as have been discovered to co-exist in Nature. And our Ideas being thus true, though not, perhaps, very exact Copies, are yet the Subjects of real (as far as we have any) Knowledge of them. (IV.iv.12)

This condition keeps our sensitive knowledge from being arbitrary or otherwise potentially ungrounded in the nature of how things are outside the mind. Locke’s worry here is that the fundamental principle governing the combination of ideas—viz., logical contradiction—is insufficient to give us knowledge of how qualities might actually combine in the world. A logically coherent combination of ideas might signify a really incoherent or incompatible combination of qualities.

Second, Locke argues that we can form ‘nominal’ essences, and thus have general knowledge of the physical world (e.g. that all gold is malleable), insofar as we keep to combinations that accord with experienced regularities. Locke bases this on the grounds that “whatever have once had an union in Nature, may be united again.” (IV.iv.12) Locke thus advocates a view according to which our knowledge of the external world, including even its very basic nature, is always subject to revision.


  1. Cf. @ayers1991, vol. II., 19. ↩︎

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