An Enquiry Concerning Human Understanding
IV. Sceptical Doubts Concerning The Operations Of The
20. All the objects of human reason or enquiry may naturally be divided into two kinds,
to wit, Relations of Ideas, and Matters of Fact. Of the first kind are the sciences of
Geometry, Algebra, and Arithmetic; and in short, every affirmation which is either
intuitively or demonstratively certain. That the square of the hypothenuse is equal to the
square of the two sides, is a proposition which expresses a relation between these figures.
That three times five is equal to the half of thirty, expresses a relation between these
numbers. Propositions of this kind are discoverable by the mere operation of thought,
without dependence on what is anywhere existent in the universe. Though there never
were a circle or triangle in nature, the truths demonstrated by Euclid would for ever retain
their certainty and evidence.
21. Matters of fact, which are the second objects of human reason, are not ascertained in
the same manner; nor is our evidence of their truth, however great, of a like nature with
the foregoing. The contrary of every matter of fact is still possible; because it can never
imply a contradiction, and is conceived by the mind with the same facility and
distinctness, as if ever so conformable to reality. That the sun will not rise to-morrow is
no less intelligible a proposition, and implies no more contradiction than the affirmation,
that it will rise. We should in vain, therefore, attempt to demonstrate its falsehood. Were
it demonstratively false, it would imply a contradiction, and could never be distinctly
conceived by the mind.
It may, therefore, be a subject worthy of curiosity, to enquire what is the nature of that
evidence which assures us of any real existence and matter of fact, beyond the present
testimony of our senses, or the records of our memory. This part of philosophy, it is
observable, has been little cultivated, either by the ancients or moderns; and therefore our
doubts and errors, in the prosecution of so important an enquiry, may be the more
excusable; while we march through such difficult paths without any guide or direction.
They may even prove useful, by exciting curiosity, and destroying that implicit faith and
security, which is the bane of all reasoning and free enquiry. The discovery of defects in
the common philosophy, if any such there be, will not, I presume, be a discouragement,
but rather an incitement, as is usual, to attempt something more full and satisfactory than
has yet been proposed to the public.
22. All reasonings concerning matter of fact seem to be founded on the relation of Cause
and Effect. By means of that relation alone we can go beyond the evidence of our
memory and senses. If you were to ask a man, why he believes any matter of fact, which
is absent; for instance, that his friend is in the country, or in France; he would give you a
reason; and this reason would be some other fact; as a letter received from him, or the
knowledge of his former resolutions and promises. A man finding a watch or any other
machine in a desert island, would conclude that there had once been men in that island.