Thomas Reid.

The works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters online

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Every point of knowledge, and every
judgment, is expressed by a proposition,
wherein something is affirmed or denied of
the subject of the proposition.

By perceiving the connection or agree-
ment of two ideas, I conceive, is meant per-
ceiving the truth of an affirmative proposi-
tion, of which the subject and predicate are
ideas. In like manner, by perceiving the
disagreement and repugnancy of any two
ideas, I conceive is meant perceiving the
truth of a negative proposition, of which
both subject and predicate are ideas. This
I take to be the only meaning the words
can bear, and it is confirmed by what Mr
Locke says in a passage already quoted in
this chapter, -that " the mind, taking its
ideas to agree or disagree, is the same as
taking any proposition to be true or false. 1 '
Therefore, if the definition of knowledge
given by Mr Locke be a just one, the sub-
ject, as well as the predicate of every pro-
position, by which any point of knowledge
is expressed, must be an idea, and can be
nothing else ; and the same must hold of
every proposition by which judgment is
expressed, as has been shewn above.

Having ascertained the meaning of this
definition of human knowledge, we are
next to consider how far it is just.

First, I would observe that, if the word

idea be taken in the meaning which it had
at first among the Pythagoreans and Pla-
tonists, and if by knowledge be meant only
abstract and general knowledge, (which I
believe Mr Locke had chiefly in his view,)
I think the proposition is true, that such
knowledge consists solely in perceiving the
truth of propositions whose subject and
predicate are ideas. [539]

By ideas here I mean things conceived
abstractly, without regard to their existence.
We commonly call them abstract notions,
abstract conceptions, abstract ideas — the
Peripatetics called them universals ; and
the Platonists, who knew no other ideas,
called them ideas without addition.

Such ideas are both subject and predicate
in every proposition which expresses ab-
stract knowledge.

The whole body of pure mathematics is
an abstract science ; and in every mathe-
matical proposition, both subject and pre-
dicate are ideas, in the senseabove explained.
Thus, when I say the side of a square is not
commensurable to its diagonal — in this
proposition the side and the diagonal of a
square are the subjects, (for, being a rela-
tive proposition, it must have two subjects.)
A square, its side, and its diagonal, are
ideas, or universals ; they are not indivi-
duals, but things predicable of many indi-
viduals. Existence is not included in their
definition, nor in the conception we form of
them. The predicate of the proposition is
commensurable, which must be an univer-
sal, as the predicate of every proposition is
so. In other branches of knowledge, many
abstract truths may be found, but, for the
most part, mixed with others that are not

I add, that I apprehend that what is strictly
called demonstrative evidence, is to be found
in abstract knowledge only. This was the
opinion of Aristotle, of Plato, and, I think,
of all the ancient philosophers ; and I be-
lieve in this they judged right. It is true,
we often meet with demonstration in astro-
mony, in mechanics, and in other branches
of natural philosophy ; but, I believe, we
shall always find that such demonstrations
are grounded upon principles of supposi-
tions, which have neither intuitive nor
demonstrative evidence. [540]

Thus, when we demonstrate that the
path of a projectile in vacuo is a parabola,
we suppose that it is acted upon with the
same force and in the same direction
through its whole path by gravity. This is
not intuitively known, nor is it demon-
strable ; and, in the demonstration, we rea-
son from the laws of motion, which are
principles not capable of demonstration,
but grounded on a different kind of evidence.

Ideas, in the sense above explained, are
creatures of the mind ; they are fabricated




by its rational powers ; we know their
nature and their essence— for they are
nothing more than they are conceived to
be ; — and, because they are perfectly known,
we can reason about them with the highest
degree of evidence.

And, as they are not things that exist,
but things conceived, they neither have
place nor time, nor are they liable to

When we say that they are in the mind,
this can mean no more but that they are
conceived by the mind, or that they are
objects of thought. The act of conceiving
them is, no doubt, in the mind ; the things
conceived have no place, because they have
not existence. Thus, a. circle, considered
abstractly, is said figuratively to be in the
mind of him that conceives it ; but in no
other sense than the city of London or the
kingdom of France is said to be in his
mind when he thinks of those objects.

Place and time belong to finite things that
exist, but not to things that are barely con-
ceived. They may be objects of concep-
tion to intelligent beings in every place and
at all times. Hence the Pythagoreans and
Platonists were led to think that they are
eternal and omnipresent. If they had ex-
istence, they must be so ; for they have no
relation to any one place or time, which
they have not to every place and to every

The natural prejudice of mankind, that
what we conceive must have existence, led
those ancient philosophers to attribute ex-
istence to ideas ; and by this they were led
into all the extravagant and mysterious
parts of their system. When it is purged
of these, I apprehend it to be the only in-
telligible and rational system concerning
ideas. [541]

I agree with them, therefore, that ideas
are immutably the same in all times and
places ; for this means no more but that a
circle is always a circle, and a square always
a square.

I agree with them that ideas are the pat-
terns or exemplars by which everything
was made that had a beginning: for an
intelligent artificer must conceive his work
before it is made ; he makes it according to
that conception ; and the thing conceived,
before it exists, can only be an idea.

I agree with them that every species of
things, considered abstractly, is an idea;
and that the idea of the species is in every
individual of the species, without division
or multiplication. This, indeed, is expressed
somewhat mysteriously, according to the
manner of the sect ; but it may easily be

Every idea is an attribute ; and it is a
common way of speaking to say, that the
attribute is in every subject of which it may
r 541-543l

truly be affirmed. Thus, to be above fifty
years of age is an attribute or idea. This
attribute may be in, or affirmed of, fifty
different individuals, and be the same in
all, without division or multiplication.

I think that not only every species, but
every genus, higher or lower, and every
attribute considered abstractly, is an idea.
These are things conceived without regard to
existence ; they are universals, and, there-
fore, ideas, according to the ancient mean-
ing of that word. [542]

It is true that, after the Platonists en-
tered into disputes with the Peripatetics, in
order to defend the existence of eternal
ideas, they found it prudent to contract the
line of defence, and maintained only that
there is an idea of every species of natural
things, but not of the genera, nor of things
artificial. They were unwilling to multiply
beings beyond what was necessary; but
in this, I think, they departed from the
genuine principles of their system.

The definition of a species is nothing
but the definition of the genus, with the
addition of a specific difference ; and the
division of things into species is the work
of the mind, as well as their division into
genera and classes. A species, a genus, an
order, a class, is only a combination of at-
tributes made by the mind, and called by
one name. There is, therefore, the same
reason for giving the name of idea to every
attribute, and to every species and genus,
whether higher or lower : these are only
more complex attributes, or combinations
of the more simple. And, though it might
be improper, without necessity, to multiply
beings which they believed to have a real
existence, yet, had they seen that ideas
are not things that exist, but things that
are conceived, they would have appre-
hended no danger nor expense from their

Simple attributes, species and genera,
lower or higher, are all things conceived
without regard to existence ; they are uni-
versals ; they are expressed by general
words ; and have an equal title to be called
by the name of ideas.

I likewise agree with those ancient phi-
losophers that ideas are the object, and the
sole object, of science, strictly so called —
that is, of demonstrative reasoning.

And, as ideas are immutable, so their
agreements and disagreements, and all their
relations and attributes, are immutable.
All mathematical truths are immutably
true. Like the ideas about which they are
conversant, they have no relation to time
or place, no dependence upon existence or
change. That the angles of a plane tri-
angle are equal to two right angles always
was, and always will be, true, though no
triangle had ever existed. [543]



£essay VI

The same may be said of all abstract
truths : on that account they hare often
been called eternal truths; and, for the
same reason, the Pythagoreans ascribed
eternity to the ideas about which they are
conversant. They may very properly be
called necessary truths ; because it is im-
possible they should not be true at all times
and in all places.

Such is the nature of all truth that can
be discovered, by perceiving the agreements
and disagreements of ideas, when we take
that word in its primitive sense. And that
Mr Locke, in his definition of knowledge,
had chiefly in his view abstract truths, we
may be led to think from the examples he
gives to illustrate it.

But there is another great class of truths,
which are not abstract and necessary, and,
therefore, cannot be perceived in the agree-
ments and disagreements of ideas. These
are all the truths we know concerning the
real existence of things — the truth of our
own existence — of the existence of other
things, inanimate, animal, and rational, and
of their various attributes and relations.

These truths may be called contingent
truths. I except only the existence and
attributes of the Supreme Being, which is
the only necessary truth I know regarding

All other beings that exist depend for
their existence, and all that belongs to it,
upon the will and power of the first cause ;
therefore, neither their existence, nor their
nature, nor anything that befalls them, is
necessary, but contingent.

But, although the existence of the Deity
be necessary, I apprehend we can only de-
duce it from contingent truths. The only
arguments for the existence of a Deity
which I am able to comprehend, are ground-
ed upon the knowledge of my own existence,
and the existence of other finite beings.
But these are contingent truths. [544]

I believe, therefore, that by perceiving
agreements and disagreements of ideas, no
contingent truth whatsoever can be known,
nor the real existence of anything, not even
our own existence, nor the existence of a
Deity, which is a necessary truth. Thus I
have endeavoured to shew what knowledge
may, and what cannot be attained, by per-
ceiving the agreements and disagreements
of ideas, when we take that word in its
primitive sense.

We are, in the next place, to consider,
whether knowledge consists in perceiving the
agreement or disagreement of ideas, taking
ideas in any of the senses in which the word
is used by Mr Locke and other modern

1. Very often the word idea is used so,
that to have the idea of anything is a peri,
phrasis for conceiving it. In this sense, an

idea is not an object of thought, it is thought
itself. It is the act of the mind by which
we conceive any object. And it is evident
that this could not be the meaning which
Mr Locke had in view in his definition of

2. A second meaning of the word idea is
that which Mr Locke gives in the intro-
duction to his Essay, when he is making an
apology for the frequent use of it : — " It be-
ing that term, I think, which serves best to
stand for whatsoever is the object of the
understanding when a man thinks, or what-
ever it is which a man can be employed
about in thinking."

By this definition, indeed, everything that
can be the object of thought is an idea.
The objects of our thoughts may, I think,
be reduced to two classes.

The first class comprehends all those
objects which we not only can think of, but
which we believe to have a real existence :
such as the Creator of all things, and all
his creatures that fall within our notice.
[545] I oan think of the sun and moon,
the earth and sea, and of the various animal,
vegetable, and inanimate productions with
which it hath pleased the bountiful Creator
to enrich our globe. I can think of myself,
of my friends and acquaintance. I think
of the author of the Essay with high esteem.
These, and such as these, are objects of the
understanding which we believe to have real

A second class of objects of the under-
standing which a man may be employed
about in thinking, are things which we either
believe never to have existed, or which we
think of without regard to their existence.

Thus, I can think of Don Quixote, of
the Island of Laputa, of Oceana, and of
Utopia, which I believe never to have ex-
isted. Every attribute, every species, and
every genus of things, considered abstractly,
without any regard to their existence or
non-existence, may be an object of the

To this second class of objects of the
understanding, the name of idea does very
properly belong, according to the primitive
sense of the word, and I have already con-
sidered what knowledge does and what
does not consist in perceiving the agree-
ments and disagreements of such ideas.

But, if we take the word idea in so ex-
tensive a sense as to comprehend, not only
the second, but also the first class of objects
of the understanding, it will undoubtedly
be true that all knowledge consists in per-
ceiving the agreements and disagreement*
of ideas : for it is impossible that there can
be any knowledge, any judgment, any
opinion, true or false, which is not employed
about the objects of the understanding.
But whatsoever is an object of the under-
[Sit, SW]



standing is an idea, according to this second
meaning of the word.

Yet I am persuaded that Mr Locke, in
his definition of knowledge, did not mean
that the word idea should extend to all those
things which we commonly consider as ob-
jects of the understanding. [546]

Though Bishop Berkeley believed that
sun, moon, and stars, and all material things,
are ideas, and nothing but ideas, Mr Locke
nowhere professes this opinion. He be-
lieved that we have ideas of bodies, but not
that bodies are ideas. In like manner, he
believed that we have ideas of minds, but
not that minds are ideas. When he in-
quired so carefully into the origin of all our
ideas, he did not surely mean to find the
origin of whatsoever may be the object of
the understanding, nor to resolve the origin
of everything that may be an object of
understanding into sensation and reflec-

3. Setting aside, therefore, the two mean-
ings of the word idea, before mentioned, as
meanings which Mr Locke could not have
in his view in the definition he gives of
knowledge, the only meaning that could be
intended in this place is that which I before
called the philosophical meaning of the
word idea, which hath a reference to the
theory commonly received about the manner
in which the mind perceives external obj ects,
and in which it remembers and conceives
objects that are not present to it. Itis avery
ancient opinion, ana has been very generally
received among philosophers, that we can-
not perceive or think of such objects im-
mediately, but by the medium of certain
images or representatives of them really
existing in the mind at the time.

To those images the ancients gave the
name of species and phantasms. Modern
philosophers have given them the name of
idea* " "Tis evident," says Mr Locke,
book iv., chap. 4, "themindknows not things
immediately, but only by the intervention
of the ideas it has of them." And in the
same paragraph he puts this question :
" How shall the mind, when it perceives
nothing but its own ideas, know that they
agree with things themselves ?" [547]

This theory I have already considered,
in treating of perception, of memory, and
of conception. The reader will there find
the reasons that lead me to think that it
has no solid foundation in reason, or in
attentive reflection upon those operations
of our minds ; that it contradicts the im-
mediate dictates of our natural faculties,
which are of higher authority than any
theory ; that it has taken its rise from the
same prejudices which led all the ancient
philosophers to think that the Deity could
not make this world without some eternal
matter to work upon, and which led the

Pythagoreans and Platonists to think that
he could not conceive the plan of the world
he was to make without eternal ideas really
existing as patterns to work by ; and that
this theory, when its necessary consequences
are fairly pursued, leads to absolute scep-
ticism, though those consequences were not
seen by most of the philosophers who have
adopted it.

I have no intention to repeat what has
before been said upon those points ; but
only, taking ideas in this sense, to make
some observations upon the definition which
Mr Locke gives of knowledge.

First, If all knowledge consists in per-
ceiving the agreements and disagreements
of ideas — that is, of representative images of
things existing in the mind — it obviously
follows that, if there be no such ideas, there
can be no knowledge. So that, if there
should be found good reason for giving up
this philosophical hypothesis, all knowledge
must go along with it.

I hope, however, it is not so : and that,
though this hypothesis, like many others,
should totter and fall to the ground, know-
ledge will continue to stand firm upon a
more permanent basis. [548]

The cycles and epicycles of the ancient
astronomers were for a thousand years
thought absolutely necessary to explain
the motions of the heavenly bodies. Yet
now, when all men believe them to have
been mere fictions, astronomy has not fallen
with them, but stands upon a more rational
foundation than before. Ideas, or images
of things existing in the mind, have, for a
longer time, been thought necessary for
explaining the operations of the understand-
ing. If they should likewise at last be
found to be fictions, human knowledge and
judgment would suffer nothing by being
disengaged from an unwieldy hypothesis.
Mr Locke surely did not look upon the ex-
istence of ideas as a philosophical hypo-
thesis. He thought that we are conscious
of their existence, otherwise he would not
have made the existence of all our know-
ledge to depend upon the existence of ideas.

Secondly, Supposing this hypothesis to
be true, I agree with Mr Locke that it is
an evident and necessary consequence that
our knowledge can be conversant about
ideas only, and must consist in perceiving
their attributes and relations. For nothing
can be more evident than this, that all
knowledge, and all judgment and opinion,
must be about things which are or may be
immediate objects of our thought. What
cannot be the object of thought, or the
object of the mind in thinking, cannot be
the object of knowledge or of opinion.

Everything we can know of any object,
must be either some attribute of the object,
or some relation it bears to some other



[essay VI.

object or objects. By the agreements and
disagreements of objects, I apprehend Mr
Locke intended to express both their attri-
butes and their relations. If ideas then be
the only objects of thought, the consequence
is necessary, that they must be the only
objects of knowledge, and all knowledge
must consist in perceiving their agreements
and disagreements — that is, their attributes
and relations.

The use I would make of this conse-
quence, is to shew that the hypothesis must
be false, from which it necessarily follows.
For if we have any knowledge of things
that are not ideas, it will follow no less
evidently, that ideas are net the only objects
of our thoughts. [549]

Mr Locke has pointed out the extent and
limits of human knowledge, in his fourth
book, with more accuracy and judgment
than any philosopher had done before ; but
he has not confined it to the agreements
and disagreements of ideas. And I cannot
help thinking that a great part of that book
is an evident refutation of the principles
laid down in the beginning of it.

Mr Locke did not believe that he himself
was an idea ; that his friends and acquaint-
ance were ideas ; that the Supreme Being,
to speak with reverence, is an idea ; or
that the sun and moon, the earth and the
sea, and other external objects of sense, are
ideas. He believed that he had some cer-
tain knowledge of all those objects. His
knowledge, therefore, did not consist solely
in perceiving the agreements and disagree-
ments of his ideas ; for, surely, to perceive
the existence, the attributes, and relations
of things, which are not ideas, is not to per-
ceive the agreements and disagreements of
ideas- And, if things which are not ideas be
objects of knowledge, they must be objects of
thought. On the contrary, if ideas be the
only objects of thought, there can be no
knowledge, either of our own existence, or
of the existence of external objects, or of
the existence of a Deity.

This consequence, as far as concerns the
existence of external objects of sense, was
afterwards deduced from the theory of ideas
by Bishop Berkeley with the clearest evi-
dence ; and that author chose rather to
adopt the consequence than to reject the
theory on which it was grounded. But,
with regard to the existence of our own
minds, of other minds, and of a Supreme
Mind, the Bishop, that he might avoid the
consequence, rejected a part of the theory,
and maintained that we can think of minds,
of their attributes and relations, without
ideas. [550]

Mr Hume saw very clearly the conse-
quences of this theory, and adopted them
in his speculative moments ; but candidly
acknowledges that, in the common busi-

ness of life, he found himself under a neces-
sity of believing with the vulgar. Hie
" Treatise of Human Nature" is the only
system to which the theory of ideas leads ;
and, in my apprehension, is, in all its parts,
the necessary consequence of that theory.

Mr Locke, however, did not see all the
consequences of that theory ; he adopted it
without doubt or examination, carried along
by the stream of philosophers that went
before him ; and his judgment and good
sense have led him to say many things, and
to believe many things, that cannot be re-
conciled to it.

He not only believed his own existence,
the existence of external things, and the
existence of a Deity ; but he has shewn
very justly how we come by the knowledge
of these existences.

It might here be expected that he should
have pointed out the agreements and dis-
agreements of ideas from which these exist-
ences are deduced ; but this is impossible,
and he has not even attempted it.

Our own existence, he observes, we know
intuitively; but this intuition is not a percep-
tion of the agreement or disagreement of
ideas ; for the subject of the proposition, 1
exist, is not an idea, but a person.

The knowledge of external objects of
sense, he observes, we can have only by sensa-
tion. This sensation he afterwards expresses
more clearly by the testimony of our senses,
which are the proper and sole judges of this
thing; whose testimony is the greatest assur-
ance we can possibly have, and to which
our faculties can attain. This is perfectly
agreeable to the common sense of mankind,
and is perfectly understood by those who
never heard of the theory of ideas. Our
senses testify immediately the existence,
and many of the attributes and relations of
external material beings ; and, by our con-

Online LibraryThomas ReidThe works of Thomas Reid, D.D.; now fully collected, with selections from his umpublished letters → online text (page 97 of 114)