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