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An Enquiry Concerning Human Understanding. (1748)

Sect. VI. Of Probability

David Hume.

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An Enquiry Concerning Human Understanding.

David Hume.

Sect. VI. Of Probability*

  * Mr. Locke divides all arguments into demonstrative and probable.
In this view, we must say, that it is only probable all men must
die, or that the sun will rise to-morrow. But to conform our
language more to common use, we ought to divide arguments into
demonstrations, proofs, and probabilities. By proofs meaning such
arguments from experience as leave no room for doubt or opposition.

  46. THOUGH there be no such thing as Chance in the world; our
ignorance of the real cause of any event has the same influence on the
understanding, and begets a like species of belief or opinion.

  There is certainly a probability, which arises from a superiority of
chances on any side; and according as this superiority encreases,
and surpasses the opposite chances, the probability receives a
proportionable encrease, and begets still a higher degree of belief or
assent to that side, in which we discover the superiority. If a die
were marked with one figure or number of spots on four sides, and with
another figure or number of spots on the two remaining sides, it would
be more probable, that the former would turn up than the latter;
though, if it had a thousand sides marked in the same manner, and only
one side different, the probability would be much higher, and our
belief or expectation of the event more steady and secure. This
process of the thought or reasoning may seem trivial and obvious;
but to those who consider it more narrowly, it may, perhaps, afford
matter for curious speculation.

  It seems evident, that, when the mind looks forward to discover
the event, which may result from the throw of such a die, it considers
the turning up of each particular side as alike probable; and this the
very nature of chance, to render all the particular events,
comprehended in it, entirely equal. But finding a greater number of
sides concur in the one event than in the other, the mind is carried
more frequently to that event, and meets it oftener, in revolving
the various possibilities or chances, on which the ultimate result
depends. This concurrence of several views in one particular event
begets immediately, by an inexplicable contrivance of nature, the
sentiment of belief, and gives that event the advantage over its
antagonist, which is supported by a smaller number of views, and
recurs less frequently to the mind. If we allow, that belief is
nothing but a firmer and stronger conception of an object than what
attends the mere fictions of the imagination, this operation may,
perhaps, in some measure, be accounted for. The concurrence of these
several views or glimpses imprints the idea more strongly on the
imagination; gives it superior force and vigour; renders its influence
on the passions and affections more sensible; and in a word, begets
that reliance or security, which constitutes the nature of belief
and opinion.

  47. The case is the same with the probability of causes, as with
that of chance. There are some causes, which are entirely uniform
and constant in producing a particular effect; and no instance has
ever yet been found of any failure or irregularity in their operation.
Fire has always burned, and water suffocated every human creature: The
production of motion by impulse and gravity is an universal law, which
has hitherto admitted of no exception. But there are other causes,
which have been found more irregular and uncertain; nor has rhubarb
always proved a purge, or opium a soporific to every one, who has
taken these medicines. It is true, when any cause fails of producing
its usual effect, philosophers ascribe not this to any irregularity in
nature; but suppose, that some secret causes, in the particular
structure of parts, have prevented the operation. Our reasonings,
however, and conclusions concerning the event are the same as if
this principle had no place. Being determined by custom to transfer
the past to the future, in all our inferences; where the past has been
entirely regular and uniform, we expect the event with the greatest
assurance, and leave no room for any contrary supposition. But where
different effects have been found to follow from causes, which are
to appearance exactly similar, all these various effects must occur to
the mind in transferring the past to the future, and enter into our
consideration, when we determine the probability of the event.
Though we give the preference to that which has been found most usual,
and believe that this effect will exist, we must not overlook the
other effects, but must assign to each of them a particular weight and
authority, in proportion as we have found it to be more or less
frequent. It is more probable, in almost every country of Europe, that
there will be frost sometime in January, than that the weather will
continue open throughout that whole month; though this probability
varies according to the different climates, and approaches to a
certainty in the more northern kingdoms. Here then it seems evident,
that, when we transfer the past to the future, in order to determine
the effect, which will result from any cause, we transfer all the
different events, in the same proportion as they have appeared in
the past, and conceive one to have existed a hundred times, for
instance, another ten times, and another once. As a great number of
views do here concur in one event, they fortify and confirm it to
the imagination, beget that sentiment which we call belief, and give
its object the preference above the contrary event, which is not
supported by an equal number of experiments, and recurs not so
frequently to the thought in transferring the past to the future.
Let any one try to account for this operation of the mind upon any
of the received systems of philosophy, and he will be sensible of
the difficulty. For my part, I shall think it sufficient, if the
present hints excite the curiosity of philosophers, and make them
sensible how defective all common theories are in treating of such
curious and such sublime subjects.

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