In his adherence to a truth, where the
demonstration by which it was at first known is forgot, though a man
may be thought rather to believe his memory than really to know, and
this way of entertaining a truth seemed formerly to me like
something between opinion and knowledge; a sort of assurance which
exceeds bare belief, for that relies on the testimony of another;- yet
upon a due examination I find it comes not short of perfect certainty,
and is in effect true knowledge. That which is apt to mislead our
first thoughts into a mistake in this matter is, that the agreement or
disagreement of the ideas in this case is not perceived, as it was
at first, by an actual view of all the intermediate ideas whereby
the agreement or disagreement of those in the proposition was at first
perceived; but by other intermediate ideas, that show the agreement or
disagreement of the ideas contained in the proposition whose certainty
we remember. For example: in this proposition, that "the three
angles of a triangle are equal to two right ones," one who has seen
and clearly perceived the demonstration of this truth knows it to be
true, when that demonstration is gone out of his mind; so that at
present it is not actually in view, and possibly cannot be
recollected: but he knows it in a different way from what he did
before.
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