ideas together, or separating
them from one another in the mind, when their certain agreement or
disagreement is not perceived, but PRESUMED to be so; which is, as the
word imports, taken to be so before it certainly appears. And if it
so unites or separates them as in reality things are, it is right
judgement.

CHAPTER XV. OF PROBABILITY.

1. Probability is the appearance of Agreement upon fallible Proofs.

As DEMONSTRATION is the showing the agreement or disagreement of two
ideas, by the intervention of one or more proofs, which have a constant,
immutable, and visible connexion one with another; so PROBABILITY is
nothing but the appearance of such an agreement or disagreement, by the
intervention of proofs, whose connexion is not constant and immutable,
or at least is not perceived to be so, but is, or appears for the most
part to be so, and is enough to induce the mind to judge the proposition
to be true or false, rather than the contrary. For example: in the
demonstration of it a man perceives the certain, immutable connexion
there is of equality between the three angles of a triangle, and those
intermediate ones which are made use of to show their equality to two
right ones; and so, by an intuitive knowledge of the agreement or
disagreement of the intermediate ideas in each step of the progress,
the whole series is continued with an evidence, which clearly shows the
agreement or disagreement of those three angles in equality to two right
ones: and thus he has certain knowledge that it is so. But another
man, who never took the pains to observe the demonstration, hearing a
mathematician, a man of credit, affirm the three angles of a triangle to
be equal to two right ones, assents to it, i.e. receives it for true: in
which case the foundation of his assent is the probability of the thing;
the proof being such as for the most part carries truth with it: the man
on whose testimony he receives it, not being wont to affirm anything
contrary to or besides his knowledge, especially in matters of this
kind: so that that which causes his assent to this proposition, that the
three angles of a triangle are equal to two right ones, that which makes
him take these ideas to agree, without knowings them to do so, is the
wonted veracity of the speaker in other cases, or his supposed veracity
in this.

2. It is to supply our Want of Knowledge.

Our knowledge, as has been shown, being very narrow, and we not happy
en