Philosiphy

515

John Locke

into the true meaning of it; but where there is not

sufficient to guide the reader, there it concerns the writer

to explain his meaning, and show in what sense he there

uses that term.

BOOK IV Of Knowledge and Probability

Chapter I Of Knowledge in General

1. Our knowledge conversant about our ideas only. 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 knowl-

edge is only conversant about them.

2. Knowledge is the perception of the agreement or

disagreement of two ideas. 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. Where this percep-

tion is, there is knowledge, and where it is not, there,

though we may fancy, guess, or believe, yet we always

come short of knowledge. For when we know that white

is not black, what do we else but perceive, that these

two ideas do not agree? When we possess ourselves with

the utmost security of the demonstration, that the three

angles of a triangle are equal to two right ones, what do

we more but perceive, that equality to two right ones

does necessarily agree to, and is inseparable from, the

three angles of a triangle?

3. This agreement or disagreement may be any of four

sorts. But to understand a little more distinctly wherein

this agreement or disagreement consists, I think we may

reduce it all to these four sorts:

I. Identity, or diversity.

II. Relation.

III. Co-existence, or necessary connexion.

IV. Real existence.

4. Of identity, or diversity in ideas. First, As to the first

sort of agreement or disagreement, viz. identity or di-

versity. It is the first act of the mind, when it has any

sentiments or ideas at all, to perceive its ideas; and so

516

Human Understanding

far as it perceives them, to know each what it is, and

thereby also to perceive their difference, and that one is

not another. This is so absolutely necessary, that with-

out it there could be no knowledge, no reasoning, no

imagination, no distinct thoughts at all. By this the

mind clearly and infallibly perceives each idea to agree

with itself, and to be what it is; and all distinct ideas to

disagree, i.e. the one not to be the other: and this it

does without pains, labour, or deduction; but at first

view, by its natural power of perception and distinction.

And though men of art have reduced this into those

general rules, What is, is, and It is impossible for the

same thing to be and not to be, for ready application in

all cases, wherein there may be occasion to reflect on it:

yet it is certain that the first exercise of this faculty is

about particular ideas. A man infallibly knows, as soon

as ever he has them in his mind, that the ideas he calls

white and round are the very ideas they are; and that

they are not other ideas which he calls red or square.

Nor can any maxim or proposition in the world make

him know it clearer or surer than he did before, and

without any such general rule. This then is the first

agreement or disagreement which the mind perceives in

its ideas; which it always perceives at first sight: and if

there ever happen any doubt about it, it will always be

found to be about the names, and not the ideas them-

selves, whose identity and diversity will always be per-

ceived, as soon and clearly as the ideas themselves are;

nor can it possibly be otherwise.

5. Of abstract relations between ideas. Secondly, the

next sort of agreement or disagreement the mind per-

ceives in any of its ideas may, I think, be called relative,

and is nothing but the perception of the relation be-

tween any two ideas, of what kind soever, whether sub-

stances, modes, or any other. For, since all distinct ideas

must eternally be known not to be the same, and so be

universally and constantly denied one of another, there

could be no room for any positive knowledge at all, if we

could not perceive any relation between our ideas, and

find out the agreement or disagreement they have one

with another, in several ways the mind takes of compar-

ing them.

517

John Locke

6. Of their necessary co-existence in substances. Thirdly,

The third sort of agreement or disagreement to be found

in our ideas, which the perception of the mind is em-

ployed about, is co-existence or non-co-existence in the

same subject; and this belongs particularly to substances.

Thus when we pronounce concerning gold, that it is

fixed, our knowledge of this truth amounts to no more

but this, that fixedness, or a power to remain in the fire

unconsumed, is an idea that always accompanies and is

joined with that particular sort of yellowness, weight,

fusibility, malleableness, and solubility in aqua regia,

which make our complex idea signified by the word gold,

7. Of real existence agreeing to any idea. Fourthly, The

fourth and last sort is that of actual real existence agree-

ing to any idea. Within these four sorts of agreement or

disagreement is, I suppose, contained all the knowledge

we have, or are capable of For all the inquiries we can

make concerning any of our ideas, all that we know or

can affirm concerning any of them, is, That it is, or is

not, the same with some other; that it does or does not

always coexist with some other idea in the same subject;

that it has this or that relation with some other idea; or

that it has a real existence without the mind. Thus, “blue

is not yellow,” is of identity. “Two triangles upon equal

bases between two parallels are equal,” is of relation. “Iron

is susceptible of magnetical impressions,” is of co-exist-

ence. “God is,” is of real existence. Though identity and

co-existence are truly nothing but relations, yet they are

such peculiar ways of agreement or disagreement of our

ideas, that they deserve well to be considered as distinct

heads, and not under relation in general; since they are

so different grounds of affirmation and negation, as will

easily appear to any one, who will but reflect on what is

said in several places of this Essay.

I should now proceed to examine the several degrees

of our knowledge, but that it is necessary first, to con-

sider the different acceptations of the word knowledge.

8. Knowledge is either actual or habitual. There are

several ways wherein the mind is possessed of truth;

each of which is called knowledge.

I. There is actual knowledge, which is the present view

the mind has of the agreement or disagreement of any

518

Human Understanding

of its ideas, or of the relation they have one to another.

II. A man is said to know any proposition, which hav-

ing been once laid before his thoughts, he evidently

perceived the agreement or disagreement of the ideas

whereof it consists; and so lodged it in his memory, that

whenever that proposition comes again to be reflected

on, he, without doubt or hesitation, embraces the right

side, assents to, and is certain of the truth of it. This, I

think, one may call habitual knowledge. And thus a

man may be said to know all those truths which are

lodged in his memory, by a foregoing clear and full per-

ception, whereof the mind is assured past doubt as of-

ten as it has occasion to reflect on them. For our finite

understandings being able to think clearly and distinctly

but on one thing at once, if men had no knowledge of

any more than what they actually thought on, they

would all be very ignorant: and he that knew most,

would know but one truth, that being all he was able to

think on at one time.

9. Habitual knowledge is of two degrees. Of habitual

knowledge there are, also, vulgarly speaking. two degrees:

First, The one is of such truths laid up in the memory

as, whenever they occur to the mind, it actually per-

ceives the relation is between those ideas. And this is in

all those truths whereof we have an intuitive knowl-

edge; where the ideas themselves, by an immediate view,

discover their agreement or disagreement one with an-

other.

Secondly, The other is of such truths whereof the

mind having been convinced, it retains the memory of

the conviction, without the proofs. Thus, a man that

remembers certainly that he once perceived the demon-

stration, that the three angles of a triangle are equal to

two right ones, is certain that he knows it, because he

cannot doubt the truth of it. In his adherence to a

truth, where the demonstration by which it was at first

known is forgot, though a man may be thought rather

to believe his memory than really to know, and this way

of entertaining a truth seemed formerly to me like some-

thing between opinion and knowledge; a sort of assur-

ance which exceeds bare belief, for that relies on the

testimony of another;—yet upon a due examination I

519

John Locke

find it comes not short of perfect certainty, and is in

effect true knowledge. That which is apt to mislead our

first thoughts into a mistake in this matter is, that the

agreement or disagreement of the ideas in this case is

not perceived, as it was at first, by an actual view of all

the intermediate ideas whereby the agreement or dis-

agreement of those in the proposition was at first per-

ceived; but by other intermediate ideas, that show the

agreement or disagreement of the ideas contained in the

proposition whose certainty we remember. For example:

in this proposition, that “the three angles of a triangle

are equal to two right ones,” one who has seen and

clearly perceived the demonstration of this truth knows

it to be true, when that demonstration is gone out of

his mind; so that at present it is not actually in view,

and possibly cannot be recollected: but he knows it in a

different way from what he did before. The agreement

of the two ideas joined in that proposition is perceived;

but it is by the intervention of other ideas than those

which at first produced that perception. He remembers,

i.e. he knows (for remembrance is but the reviving of

some past knowledge) that he was once certain of the

truth of this proposition, that the three angles of a

triangle are equal to two right ones. The immutability of

the same relations between the same immutable things is

now the idea that shows him, that if the three angles of

a triangle were once equal to two right ones, they will

always be equal to two right ones. And hence he comes

to be certain, that what was once true in the case, is

always true; what ideas once agreed will always agree;

and consequently what he once knew to be true, he will

always know to be true; as long as he can remember that

he once knew it. Upon this ground it is, that particular

demonstrations in mathematics afford general knowledge.

If then the perception, that the same ideas will eternally

have the same habitudes and relations, be not a sufficient

ground of knowledge, there could be no knowledge of

general propositions in mathematics; for no mathemati-

cal demonstration would be any other than particular:

and when a man had demonstrated any proposition con-

cerning one triangle or circle, his knowledge would not

reach beyond that particular diagram. If he would extend

520

Human Understanding

it further, he must renew his demonstration in another

instance, before he could know it to be true in another

like triangle, and so on: by which means one could never

come to the knowledge of any general propositions. No-

body, I think, can deny, that Mr. Newton certainly knows

any proposition that he now at any time reads in his

book to be true; though he has not in actual view that

admirable chain of intermediate ideas whereby he at first

discovered it to be true. Such a memory as that, able to

retain such a train of particulars, may be well thought

beyond the reach of human faculties, when the very dis-

covery, perception, and laying together that wonderful

connexion of ideas, is found to surpass most readers’ com-

prehension. But yet it is evident the author himself knows

the proposition to be true, remembering he once saw the

connexion of those ideas; as certainly as he knows such a

man wounded another, remembering that he saw him

run him through. But because the memory is not always

so clear as actual perception, and does in all men more or

less decay in length of time, this, amongst other differ-

ences, is one which shows that demonstrative knowledge

is much more imperfect than intuitive, as we shall see in

the following chapter.

Chapter II Of the Degrees of our Knowledge

1. Of the degrees, or differences in clearness, of our

knowledge: 1. Intuitive. All our knowledge consisting,

as I have said, in the view the mind has of its own ideas,

which is the utmost light and greatest certainty we,

with our faculties, and in our way of knowledge, are

capable of, it may not be amiss to consider a little the

degrees of its evidence. The different clearness of our

knowledge seems to me to lie in the different way of

perception the mind has of the agreement or disagree-

ment of any of its ideas. For if we will reflect on our own

ways of thinking, we will find, that sometimes the 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 knowl-

edge. For in this the mind is at no pains of proving or