certainty from their supposed causes. Some events are found, in all
countries and all ages, to have been constantly conjoined together:
Others are found to have been more variable, and sometimes to disappoint
our expectations; so that, in our reasonings concerning matter of fact,
there are all imaginable degrees of assurance, from the highest
certainty to the lowest species of moral evidence.

A wise man, therefore, proportions his belief to the evidence. In such
conclusions as are founded on an infallible experience, he expects the
event with the last degree of assurance, and regards his past experience
as a full _proof_ of the future existence of that event. In other
cases, he proceeds with more caution: He weighs the opposite
experiments: He considers which side is supported by the greater number
of experiments: to that side he inclines, with doubt and hesitation; and
when at last he fixes his judgement, the evidence exceeds not what we
properly call _probability_. All probability, then, supposes an
opposition of experiments and observations, where the one side is found
to overbalance the other, and to produce a degree of evidence,
proportioned to the superiority. A hundred instances or experiments on
one side, and fifty on another, afford a doubtful expectation of any
event; though a hundred uniform experiments, with only one that is
contradictory, reasonably beget a pretty strong degree of assurance. In
all cases, we must balance the opposite experiments, where they are
opposite, and deduct the smaller number from the greater, in order to
know the exact force of the superior evidence.

88. To apply these principles to a particular instance; we may observe,
that there is no species of reasoning more common, more useful, and even
necessary to human life, than that which is derived from the testimony
of men, and the reports of eye-witnesses and spectators. This species of