In both of his two books on the Human Understanding, Hume had, I think,
one main general object. He tells us that it was his object to discover
"the extent and force of human understanding," to give us "an exact
analysis of its powers and capacity." And we may, I think, express
what he meant by this in the following way. He plainly held (as we
all do) that some men sometimes entertain opinions which they cannot
know to be true. And he wished to point out what characteristics are
possessed by those of our opinions which we _can_ know to be true, with
a view of persuading us that any opinion which does _not_ possess any
of these characteristics is of a kind which we _cannot_ know to be so.
He thus tries to lay down certain rules to the effect that the _only_
propositions which we can, any of us, know to be true are of certain
definite kinds. It is in this sense, I think, that he tries to define
the limits of human understanding.
With this object he, first of all, divides all the propositions,
which we can even so much as conceive, into two classes. They are
all, he says, either propositions about "relations of ideas" or else
about "matters of fact." By propositions about "relations of ideas"
he means such propositions as that twice two are four, or that black
differs from white; and it is, I think, easy enough to see, though
by no means easy to define, what kind of propositions it is that
he means to include in this division. They are, he says, the only
kind of propositions with regard to which we can have "intuitive" or
"demonstrative" certainty. But the vast majority of the propositions
in which we believe and which interest us most, belong to the other
division: they are propositions about "matters of fact." And these
again he divides into two classes. So far as his words go, this latter
division is between "matters of fact, beyond the present testimony of
our senses, or the records of our memory," on the one hand, and matters
of fact for which we _have_ the evidence of our memory or senses, on
the other. But it is, I think, quite plain that these words do not
represent quite accurately the division which he really means to make.
He plainly intends to reckon along with facts for which we have the
evidence of our _senses_ all facts for which we have the evidence
of _direct observation_--such facts, for instance, as those which I
observe when I observe that I am angry or afraid, and which cannot be
strictly said to be apprehended by my _senses._ The division, then,
which he really intends to make is (to put it quite strictly) into the
two classes--(1) propositions which assert some matter of fact which I
am (in the strictest sense) _observing_ at the moment, or which I have
so observed in the past and now remember; and (2) propositions which
assert any matter of fact which I am not now observing and never have
observed, or, if I have, have quite forgotten.
We have, then, the three classes--(1) propositions which assert
"relations of ideas"; (2) propositions which assert "matters of fact"
for which we have the evidence of direct observation or personal
memory; (3) propositions which assert "matters of fact_"_ for which
we have _not_ this evidence. And as regards propositions of the first
two classes, Hume does not seem to doubt our capacity for knowledge.
He does not doubt that we can know _some_ (though, of course, not
_all)_ propositions about "relations of ideas" to be true; he never
doubts, for instance, that we can know that twice two are four. And
he generally assumes also that each of us can know the truth of
_all_ propositions which merely assert some matter of fact which we
ourselves are, in the strictest sense, directly observing, or which
we have so observed and now remember. He does, indeed, in one place,
suggest a doubt whether our memory is _ever_ to be implicitly trusted,
but he generally assumes that it _always_ can. It is with regard to
propositions of the third class that he is chiefly anxious to determine
which of them (if any) we can know to be true and which not. In what
cases can any man know any matter of fact which he himself has not
directly observed? It is Hume's views on this question which form, I
think, the main interest of his philosophy.
He proposes, first of all, by way of answer to it, a rule, which may,
I think, be expressed as follows: No man, he says, can ever know any
matter of fact, which he has not himself observed, unless he can know
that it is connected by "the relation of cause and effect," with some
fact which he _has_ observed. And no man can ever know that any two
facts are connected by this relation, except by the help of his own
past _experience._ In other words, if I am to know any fact, A, which
I have not myself observed, my past experience must give me some
foundation for the belief that A is causally connected with some fact,
B, which I have observed. And the only kind of past experience which
can give me any foundation for such a belief is, Hume seems to say,
as follows: I must, he says, have found _facts like_ A "constantly
conjoined" in the past with _facts like_ B. This is what he _says;_
but we must not, I think, press his words too strictly. I may, for
instance, know that A is _probably_ a fact, even where the conjunction
of facts like it with facts like B has not been quite constant. Or
instead of observing facts like A conjoined with facts like B, I may
have observed a whole series of conjunctions--for instance, between A
and C, C and D, D and E, and E and B; and such a series, however long,
will do quite as well to establish a causal connection between A and B,
as if I had directly observed conjunctions between A and B themselves.
Such modifications as this, Hume would, I think, certainly allow. But,
allowing for them, his principle is, I think, quite clear. I can, he
holds, never know any fact whatever, which I have not myself observed,
unless I have observed similar facts in the past and have observed that
they were "conjoined" (directly or indirectly) with facts similar to
some fact which I do now observe or remember. In this sense, he holds,
_all_ our knowledge of facts, beyond the reach of our own observation,
is founded on _experience._
This is Hume's primary principle. But what consequences does he
think will follow from it, as to the kind of facts, beyond our own
observation, which we can know? We may, I think, distinguish three
entirely different views as to its consequences, which he suggests in
different parts of his work.
In the first place, where he is specially engaged in explaining this
primary principle, he certainly seems to suppose that all propositions
of the kind, which we assume most universally in everyday life, may be
founded on experience in the sense required. He supposes that we have
this foundation in experience for such beliefs as that "a stone will
fall, or fire burn"; that Julius Caesar was murdered; that the sun will
rise to-morrow; that all men are mortal He speaks as if experience did
not merely render such beliefs probable, but actually _proved_ them
to be true. The "arguments from experience" in their favour are, he
says, such as "leave no room for doubt or opposition." The only kinds
of belief, which he definitely mentions as _not_ founded on experience,
are "popular superstitions" on the one hand, and certain religious and
philosophical beliefs, on the other. He seems to suppose that a few (a
very few) religious beliefs may, perhaps, be founded on experience.
But as regards most of the specific doctrines of Christianity, for
example, he seems to be clear that they are not so founded. The belief
in miracles is not founded on experience; nor is the philosophical
belief that every event is caused by the direct volition of the Deity.
In short, it would seem, that in this doctrine that our knowledge of
unobserved facts is confined to such as are "founded on experience,"
he means to draw the line very much where it is drawn by the familiar
doctrine which is called "Agnosticism." We can know such facts as
are asserted in books on "history, geography or astronomy," or on
"politics, physics and chemistry," because such assertions may be
"founded on experience"; but we cannot know the greater part of the
facts asserted in books "of divinity or school metaphysics," because
such assertions have no foundation in experience.
This, I think, was clearly one of Hume's views. He meant to fix the
limits of our knowledge at a point which would _exclude_ most religious
propositions and a great many philosophical ones, as incapable of being
known; but which would _include_ all the other kinds of propositions,
which are most universally accepted by common-sense, as capable of
being known. And he thought that, so far as matters of fact beyond the
reach of our personal observation are concerned, this point coincided
with that at which the possibility of "foundation on experience"
ceases.
But, if we turn to another part of his work, we find a very different
view suggested. In a quite distinct section of both his books, he
investigates the beliefs which we entertain concerning the existence
of "external objects." And he distinguishes two different kinds of
belief which may be held on this subject. "Almost all mankind, and
philosophers themselves, for the greatest part of their lives,"
believe, he says, that "the very things they feel and see" _are_
external objects, in the sense that they continue to exist, even when
we cease to feel or see them. Philosophers, on the other hand, have
been led to reject this opinion and to suppose (when they reflect)
that what we actually perceive by the senses never exists except when
we perceive it, but that there are other external objects, which do
exist independently of us, and which _cause_ us to perceive what we do
perceive. Hume investigates both of these opinions, at great length in
the _Treatise_, and much more briefly in the _Enquiry_, and comes to
the conclusion, in both books, that neither of them can be "founded
on experience," in the sense he has defined. As regards the first of
them, the vulgar opinion, he does seem to admit in the _Treatise_ that
it is, in a sense, founded on experience; but not, he insists, in
the sense defined. And he seems also to think that, apart from this
fact, there are conclusive reasons for holding that the opinion cannot
be true. And as regards the philosophical opinion, he says that any
belief in external objects, which we never perceive but which cause our
perceptions, cannot possibly be founded on experience, for the simple
reason that if it were, we should need to have directly observed some
of these objects and their "conjunction" with what we do perceive,
which _ex hypothesi,_ we cannot have done, since we never do directly
observe any external object.
Hume, therefore, concludes, in this part of his work, that we cannot
know of the existence of any "external object" whatever. And though
in all that he says upon this subject, he is plainly thinking only of
_material_ objects, the principles by which he tries to prove that we
cannot know these must, I think, prove equally well that we cannot
know any "external object" whatever--not even the existence of any
other human mind. His argument is: We cannot directly observe any
object whatever, except such as exist only when we observe them; we
cannot, therefore, observe any "constant conjunctions" except between
objects of this kind: and hence we can have no foundation in experience
for any proposition which asserts the existence of any other kind of
object, and cannot, therefore, know any such proposition to be true.
And this argument must plainly apply to all the feelings, thoughts and
perceptions of other men just as much as to material objects. I can
never know that any perception of mine, or anything which I do observe,
must have been caused by any other man, because I can never directly
observe a "constant conjunction" between any other man's thoughts
or feelings or intentions and anything which I directly observe: I
cannot, therefore, know that any other man ever had any thoughts or
feelings--or, in short, that any man beside myself ever existed. The
view, therefore, which Hume suggests in this part of his work, flatly
contradicts the view which he at first seemed to hold. He now says we
_cannot_ know that a stone will fall, that fire will burn, or the sun
will rise to-morrow. All that I can possibly know, according to his
present principles, is that _I shall see_ a stone fall, shall feel the
fire burn, shall see the sun rise to-morrow. I cannot even know that
any other men will see these things; for I cannot know that any other
men exist. For the same reason, I cannot know that Julius Caesar
was murdered, or that all men are mortal. For these are propositions
asserting "external" facts--facts which don't exist only at the moment
when I observe them; and, according to his present doctrine, I cannot
possibly know any such proposition to be true. No man, in short, can
know any proposition about "matters of fact" to be true, except such
as merely assert something about _his own_ states of mind, past,
present or future--about these or about what _he himself_ has directly
observed, is observing, or will observe.
Here, therefore, we have a very different view suggested, as to the
limits of human knowledge. And even this is not all. There is yet a
third view, inconsistent with both of these, which Hume suggests in
some parts of his work.
So far as we have yet seen, he has not in any way contradicted his
original supposition that we can know _some_ matters of fact, which we
have never ourselves observed. In the second theory, which I have just
stated, he does not call in question the view that I can know all such
matters of fact as I know to be causally connected with facts which
I have observed, nor the view that I can know some facts to be thus
causally connected. All that he has done is to question whether I can
know any _external_ fact to be causally connected with anything which
I observe; he would still allow that I may be able to know that future
states of my own, or past states, which I have forgotten, are causally
connected with those which I now observe or remember; and that I may
know therefore, in some cases, what I shall experience in the future,
or have experienced in the past but have now forgotten. But in some
parts of his work he does seem to question whether any man can know
even as much as this: he seems to question whether we can ever know any
fact whatever to be causally connected with any other fact. For, after
laying it down, as we saw above, that we cannot know any fact, A, to be
causally connected with another, B, unless we have experienced in the
past a constant conjunction between facts like A and facts like B, he
goes on to ask what foundation we have for the conclusion that A and B
_are_ causally connected, even when we _have_ in the past experienced
a constant conjunction between them. He points out that from the fact
that A has been constantly conjoined with B in the past, it does not
follow that it ever will be so again. It does not follow, therefore,
that the two really are causally connected in the sense that, when the
one occurs, the other _always_ will occur also. And he concludes, for
this and other reasons, that _no argument_ can assure us that, because
they have been constantly conjoined in the past, therefore they really
are causally connected. What, then, he asks, is the foundation for such
an inference? _Custom,_ he concludes, is the only foundation. It is
nothing but custom which induces us to believe that, because two facts
have been constantly conjoined on many occasions, therefore they will
be so on _all_ occasions. We have, therefore, no better foundation
than custom for any conclusion whatever as to facts which we have not
observed. And can we be said really to _know_ any fact, for which we
have no better foundation than this? Hume himself, it must be observed,
never says that we can't. But he has been constantly interpreted as if
the conclusion that we can't really know any one fact to be causally
connected with any other, did follow from this doctrine of his. And
there is, I think, certainly much excuse for this interpretation in the
tone in which he speaks. He does seem to suggest that a belief which is
_merely_ founded on custom, can scarcely be one which we _know_ to be
true. And, indeed, he owns himself that, when he considers that this
is our only foundation for any such belief, he is sometimes tempted
to doubt whether we do know any fact whatever, except those which we
directly observe. He does, therefore, at least suggest the view that
every man's knowledge is entirely confined to those facts, which he is
directly observing at the moment, or which he has observed in the past,
and now remembers.
We see, then, that Hume suggests, at least, three entirely different
views as to the consequences of his original doctrine. His original
doctrine was that, as regards matters of fact beyond the reach of our
own actual observation, the knowledge of each of us is strictly limited
to those for which we have a basis in our own experience. And his first
view as to the consequences of this doctrine was that it does show us
to be incapable of knowing a good many religious and philosophical
propositions, which many men have claimed that they knew; but that it
by no means denies our capacity of knowing the vast majority of facts
beyond our own observation, which we all commonly suppose that we know.
His second view, on the other hand, is that it cuts off at once all
possibility of our knowing the vast majority of these facts; since he
implies that we cannot have any basis in experience for asserting any
_external_ fact whatever--any fact, that is, except facts relating to
our own actual past and future observations. And his third view is more
sceptical still, since it suggests that we cannot really know any fact
whatever, beyond the reach of our present observation or memory, even
where we _have_ a basis in experience for such a fact: it suggests that
experience cannot ever let us _know_ that any two things are causally
connected, and therefore that it cannot give us _knowledge_ of any fact
based on this relation.
What are we to think of these three views, and of the original
doctrine from which Hume seems to infer them?
As regards the last two views, it may perhaps be thought that they
are too absurd to deserve any serious consideration. It is, in fact,
absurd to suggest that I do not know any external facts whatever;
that I do not know, for instance, even that there are any men beside
myself. And Hume himself, it might seem, does not seriously expect or
wish us to accept these views. He points out, with regard to all such
excessively sceptical opinions that we cannot continue to believe them
for long together--that, at least, we cannot, for long together, avoid
believing things flatly inconsistent with them. The philosopher may
believe, when he is philosophising, that no man knows of the existence
of any other man or of any material object; but at other times he
will inevitably believe, as we all do, that he does know of the
existence of this man and of that, and even of this and that material
object. There can, therefore, be no question of making all our beliefs
consistent with such views as this of never believing anything that is
inconsistent with them. And it may, therefore, seem useless to discuss
them. But in fact, it by no means follows that, because we are not able
to adhere consistently to a given view, therefore that view is false;
nor does it follow that we may not sincerely believe it, whenever we
are philosophising, even though the moment we cease to philosophise, or
even before, we may be forced to contradict it. And philosophers do,
in fact, sincerely believe such things as this--things which flatly
contradict the vast majority of the things which they believe at other
times. Even Hume, I think, does sincerely wish to persuade us that
we cannot know of the existence of external material objects--that
this is a philosophic truth, which we ought, if we can, so long as we
are philosophising, to believe. Many people, I think, are certainly
tempted, in their philosophic moments, to believe such things; and,
since this is so, it is, I think, worth while to consider seriously
what arguments can be brought against such views. It is worth while to
consider whether they are views which we ought to hold as philosophical
opinions, even if it be quite certain that we shall never be able to
make the views which we entertain at other times consistent with them.
And it is the more worth while, because the question how we can prove
or disprove such extreme views as these, has a bearing on the question
how we can, in any case whatever, prove or disprove that we do really
_know,_ what we suppose ourselves to know.
What arguments, then, are there for or against the extreme view that
no man can know any external fact whatever; and the still more extreme
view that no man can know any matter of fact whatever, except those
which he is directly observing at the moment, or has observed in the
past and now remembers?
It may be pointed out, in the first place, that, if these views are
true, then at least no man can possibly know them to be so. What
these views assert is that I cannot know any external fact whatever.
It follows, therefore, that I cannot know that there are any other
men, beside myself, and that they are like me in this respect. Any
philosopher who asserts positively that other men, equally with
himself, are incapable of knowing any external facts, is, in that very
assertion, contradicting himself, since he implies that he _does_ know
a great many facts about the knowledge of other men. No one, therefore,
can be entitled to assert positively that human knowledge is limited
in this way, since, in asserting it positively, he is implying that
his own knowledge is not so limited. It cannot be proper, even in our
philosophic moments, to take up such an attitude as this.
No one, therefore, can know positively that men in general, are
incapable of knowing external facts. But still, although we cannot
_know_ it, it remains possible that the view should be a true one. Nay,
more, it remains possible that a man should know that _he himself_
is incapable of knowing any external facts, and that, _if_ there are
any other men whose faculties are only similar to his own, they also
must be incapable of knowing any. The argument just used obviously
does not apply against such a position as this. It only applies
against the position that men in general positively are incapable
of knowing external facts: it does not apply against the position
that the philosopher himself is incapable of knowing any, or against
the position that there are _possibly_ other men in the same case,
and that, if their faculties are similar to the philosopher's, they
certainly would be in it. I do not contradict myself by maintaining
positively that _I_ know no external facts, though I do contradict
myself if I maintain that I am only one among other men, and that no
man knows any external facts. So far, then, as Hume merely maintains
that _he_ is incapable of knowing any external facts, and that there
_may_ be other men like him in this respect, the argument just used is
not valid against his position. Can any conclusive arguments be found
against it?
It seems to me that such a position must, in a certain sense, be quite
incapable of disproof. So much must be granted to any sceptic who feels
inclined to hold it. Any valid argument which can be brought against
it must be of the nature of a _petitio principii:_ it must beg the
question at issue. How is the sceptic to prove to himself that he does
know any external facts? He can only do it by bringing forward some
instance of an external fact, which he does know; and, in assuming
that he does know this one, he is, of course, begging the question. It
is therefore quite impossible for any one to _prove,_ in one strict
sense of the term, that he does know any external facts. I can only
prove that I do, by assuming that in some particular instance, I
actually do know one. That is to say, the so-called proof must assume
the very thing which it pretends to prove. The only proof that we do
know external facts lies in the simple fact that we do know them. And
the sceptic can, with perfect internal consistency, deny that he does
know any. But it can, I think, be shown that he has no reason for
denying it. And in particular it may, I think, be easily seen that the
arguments which Hume uses in favour of this position have no conclusive
force.
To begin with, his arguments, in both cases, depend upon the two
original assumptions, (1) that we cannot know any fact, which we
have not observed, unless we know it to be causally connected with
some fact which we have observed, and (2) that we have no reason for
assuming any causal connection, except where we have experienced some
instances of conjunction between the two facts connected. And both of
these assumptions may, of course, be denied. It is just as easy to deny
them, as to deny that I do know any external facts. And, if these two
assumptions did really lead to the conclusion that I cannot know any,
it would, I think, be proper to deny them: we might fairly regard the
fact that they led to this absurd conclusion as disproving them. But,
in fact, I think it may be easily seen that they do not lead to it.
Let us consider, first of all, Hume's most sceptical argument (the
argument which he merely suggests). This argument suggests that, since
our only reason for supposing two facts to be causally connected is
that we have found them constantly conjoined in the past, and since
it does not follow from the fact that they have been conjoined ever
so many times, that they _always_ will be so, therefore we cannot
_know_ that they always will be so, and hence cannot know that they are
causally connected. But obviously the conclusion does not follow. We
must, I think, grant the premiss that, from the fact that two things
have been conjoined, no matter how often, it does not strictly _follow_
that they _always_ are conjoined. But it by no means follows from this
that we may not _know_ that, as a matter of fact, when two things are
conjoined sufficiently often, they are also _always_ conjoined. We
may quite well _know_ many things which do not logically follow from
anything else which we know. And so, in this case, we may _know_ that
two things are causally connected, although this does not logically
follow from our past experience, nor yet from anything else that we
know. And, as for the contention that our belief in causal connections
is merely based on _custom,_ we may, indeed, admit that custom would
not be a sufficient _reason_ for concluding the belief to be true.
But the mere fact (if it be a fact) that the belief is only caused by
custom, is also no sufficient reason for concluding that we can _not_
know it to be true. Custom _may_ produce beliefs, which we do know to
be true, even though it be admitted that it does not _necessarily_
produce them.
And as for Hume's argument to prove that we can never know any
_external_ object to be causally connected with anything which we
actually observe, it is, I think, obviously fallacious. In order
to prove this, he has, as he recognises, to disprove both of two
theories. He has, first of all, to disprove what he calls the vulgar
theory--the theory that we can know the very things which we see or
feel to be external objects; that is to say, can know that these very
things exist at times when we do not observe them. And even here, I
think, his arguments are obviously inconclusive. But we need not stay
to consider them, because, in order to prove that we cannot know any
external objects, he has also to disprove what he calls the philosophic
theory--the theory that we can know things which we do observe, to be
caused by external objects which we never observe. If, therefore, his
attempt to disprove this theory fails, his proof that we cannot know
any external objects also fails; and I think it is easy to see that
his disproof does fail. It amounts merely to this: That we cannot, _ex
hypothesi,_ ever observe these supposed external objects, and therefore
cannot observe them to be constantly conjoined with any objects which
we do observe. But what follows from this? His own theory about the
knowledge of causal connection is not that in order to know A to be the
cause of B, we must have observed A _itself_ to be conjoined with B;
but only that we must have observed objects _like_ A to be constantly
conjoined with objects _like_ B. And what is to prevent an external
object from being _like_ some object which we have formerly observed?
Suppose I have frequently observed a fact _like A_ to be conjoined with
a fact _like_ B: and suppose I now observe B, on an occasion when I do
not observe anything like A. There is no reason, on Hume's principles,
why I should not conclude that A does exist on this occasion, even
though I do not observe it; and that it is, therefore, an external
object. It will, of course, differ from any object which I have ever
observed, in respect of the simple fact that it is _not_ observed by
me, whereas they were. There is, therefore, this one respect in which
it must be _unlike_ anything which I have ever observed. But Hume has
never said anything to show that unlikeness in this single respect is
sufficient to invalidate the inference. It may quite well be like
objects which I have observed in all other respects; and this degree
of likeness may, according to his principles, be quite sufficient to
justify us in concluding its existence. In short, when Hume argues that
we cannot possibly learn by experience of the existence of any external
objects, he is, I think, plainly committing the fallacy of supposing
that, because we cannot, _ex hypothesis_ have ever observed any object
which actually is "external," therefore we can never have observed
any object _like_ an external one. But plainly we may have observed
objects like them in all respects except the single one that these
have been observed whereas the others have not. And even a less degree
of likeness than this would, according to his principles, be quite
sufficient to justify an inference of causal connection.
Hume does not, therefore, bring forward any arguments at all sufficient
to prove either that he cannot know any one object to be causally
connected with any other or that he cannot know any external fact. And,
indeed, I think it is plain that no conclusive argument could possibly
be advanced in favour of these positions. It would always be at least
as easy to deny the argument as to deny that we do know external facts.
We may, therefore, each one of us, safely conclude that we do know
external facts; and, if we do, then there is no reason why we should
not also know that other men do the same. There is no reason why we
should not, in this respect, make our philosophical opinions agree with
what we necessarily believe at other times. There is no reason why I
should not confidently assert that I do really _know_ some external
facts, although I cannot prove the assertion except by simply assuming
that I do. I am, in fact, as certain of this as of anything; and as
reasonably certain of it. But just as I am certain that I do know
_some_ external facts, so I am also certain that there are others which
I do not know. And the question remains: Does the line between the
two fall, where Hume says it falls? Is it true that the only external
facts I know are facts for which I have a basis in my own experience?
And that I cannot know any facts whatever, beyond the reach of my own
observation and memory, except those for which I have such a basis?
This, it seems to me, is the most serious question which Hume raises.
And it should be observed that his own attitude towards it is very
different from his attitude towards the sceptical views which we
have just been considering. These sceptical views he did not expect
or wish us to accept, except in philosophic moments. He declares
that we cannot, in ordinary life, avoid believing things which are
inconsistent with them; and, in so declaring, he, of course, implies
incidentally that they are false: since he implies that he himself
has a great deal of knowledge as to what we can and cannot believe in
ordinary life. But, as regards the view that our knowledge of matters
of fact beyond our own observation is entirely confined to such as are
founded on experience, he never suggests that it is impossible that
all our beliefs should be consistent with this view, and he does seem
to think it eminently desirable that they should be. He declares that
any assertion with regard to such matters, which is not founded on
experience, can be nothing but "sophistry and illusion"; and that all
books which are composed of such assertions should be "committed to the
flames." He seems, therefore, to think that here we really have a test
by which we may determine what we should or should not believe, on all
occasions: any view on such matters, for which we have no foundation in
experience, is a view which we cannot know to be even probably true,
and which we should _never_ accept, if we can help it. Is there any
justification for this strong view?
It is, of course, abstractly possible that we do really know, _without_
the help of experience, some matters of fact, which we never have
observed. Just as we know matters of fact, which we _have_ observed,
without the need of any further evidence, and just as we know, for
instance, that 2+2=4, without the need of any proof, it is possible
that we may know, directly and immediately, without the need of any
basis in experience, some facts which we never have observed. This
is certainly possible, in the same sense in which it is possible
that I do not really know any external facts: no conclusive disproof
can be brought against either position. We must make assumptions as
to what facts we do know and do not know, before we can proceed to
discuss whether or not all of the former are based on experience; and
none of these assumptions can, in the last resort, be conclusively
proved. We may offer one of them in proof of another; but it will
always be possible to dispute the one which we offer in proof. But
there are, in fact, certain kinds of things which we universally
assume that we do know or do not know, just as we assume that we do
know some external facts; and if among all the things which we know
as certainly as this, there should turn out to be none for which we
have no basis in experience, Hume's view would I think, be as fully
proved as it is capable of being. The question is: Can it be proved
in this sense? Among all the facts beyond our own observation, which
we know most certainly, are there any which are certainly not based
upon experience? For my part, I confess, I cannot feel certain what
is the right answer to this question: I cannot tell whether Hume was
right or wrong. But if he was wrong--if there are any matters of fact,
beyond our own observation, which we know for certain, and which yet
we know directly and immediately, without any basis in experience, we
are, I think, faced with an eminently interesting problem. For it is,
I think, as certain as anything can be that there are _some_ kinds of
facts with regard to which Hume was right--that there are _some_ kinds
of facts which we cannot know without the evidence of experience.
I could not know, for instance, without some such evidence, such a
fact as that Julius Caesar was murdered. For such a fact I must, in
the first instance, have the evidence of other persons; and if I am
to know that their evidence is trustworthy, I must have some ground
in experience for supposing it to be so. There are, therefore, some
kinds of facts which we cannot know without the evidence of experience
and observation. And if it is to be maintained that there are others,
which we can know without any such evidence, it ought to be pointed out
exactly what kind of facts these are, and in what respects they differ
from those which we cannot know without the help of experience. Hume
gives us a very clear division of the kinds of propositions which we
can know to be true. There are, first of all, some propositions which
assert "relations of ideas "; there are, secondly, propositions which
assert "matters of fact" which we ourselves are actually observing, or
have observed and now remember; and there are, thirdly, propositions
which assert "matters of fact" which we have never actually observed,
but for believing in which we have some foundation in our past
observations. And it is, I think, certain that some propositions, which
we know as certainly as we know anything, do belong to each of these
three classes. I know, for instance, that twice two are four; I know
by direct observation that I am now seeing these words, that I am
writing, and by memory that this afternoon I saw St. Paul's; and I know
also that Julius Caesar was murdered, and I have some foundation in
experience for this belief, though I did not myself witness the murder.
Do any of those propositions, which we know as certainly as we know
these and their like, _not_ belong to either of these three classes?
Must we add a fourth class consisting of propositions which resemble
the two last, in respect of the fact that they do assert "matters of
fact," but which differ from them, in that we know them neither by
direct observation nor by memory, nor yet as a result of previous
observations? There may, perhaps, be such a fourth class; but, if there
is, it is, I think, eminently desirable that it should be pointed out
exactly what propositions they are which we do know in this way; and
this, so far as I know, has never yet been done, at all clearly, by any
philosopher.