astonished at the operations
of their own reason, at the same time, that they admire the instinct
of animals, and find a difficulty in explaining it, merely because it
cannot be reducd tothe very same principles. To consider the matter
aright, reason is nothing but a wonderful and unintelligible instinct in
our souls, which carries us along a certain train of ideas, and endows
them with particular qualities, according to their particular situations
and relations. This instinct, it is true, arises from past observation
and experience; but can any one give the ultimate reason, why past
experience and observation produces such an effect, any more than why
nature alone shoud produce it? Nature may certainly produce whatever
can arise from habit: Nay, habit is nothing but one of the principles of
nature, and derives all its force from that origin.

PART IV. OF THE SCEPTICAL AND OTHER SYSTEMS OF PHILOSOPHY.

SECT. I. OF SCEPTICISM WITH REGARD TO REASON.

In all demonstrative sciences the rules are certain and infallible; but
when we apply them, our fallible said uncertain faculties are very apt
to depart from them, and fall into error. We must, therefore, in every
reasoning form a new judgment, as a check or controul on our first
judgment or belief; and must enlarge our view to comprehend a kind of
history of all the instances, wherein our understanding has deceived us,
compared with those, wherein its testimony was just and true. Our reason
must be considered as a kind of cause, of which truth is the natural
effect; but such-a-one as by the irruption of other causes, and by the
inconstancy of our mental powers, may frequently be prevented. By this
means all knowledge degenerates into probability; and this probability
is greater or less, according to our experience of the veracity or
deceitfulness of our understanding, and according to the simplicity or
intricacy of the question.

There is no Algebraist nor Mathematician so expert in his science, as to
place entire confidence in any truth immediately upon his discovery of
it, or regard it as any thing, but a were probability. Every time he
runs over his proofs, his confidence encreases; but still more by the
approbation of his friends; and is raised to its utmost perfection by
the universal assent and applauses of the learned world. Now it is
evident, that this gradual encrease of assurance is nothing but the
addition of new probabilities, and is derived fr