Essays on the Foundation and Publication of Opinions, and on other Subjects. Essays of the Pursuit of Truth, on the Progress of Knowledge, and the Fundamental Principles of all Evidence and Expectation. [pp. 394-428]

The Formation of Opinions,
the facts attested: they may imply that the same causes do not
always produce the same effects; but this would be to contradict a plain axiom of common sense, confirmed by universal
experience. Here, then, we have complete evidence on both
sides of a proposition; and of course we can believe in neither.
The rational mind, in such circumstances, can neither believe
nor disbelieve; it must remain neutral. But our ingenious
author, after bringing us to this apparent equipoise of evidence, by which all assent is rendered impossible, affords us
some relief, by discovering that the evidence from testimony
never can be as convincing as that which we have for the uniformity of causation. "The causes of testimony," he observes, "or in other words, those considerations which operate
on the mind of the witnesses, cannot be always ascertained;
and as we are uncertain as to the causes in operation, we cannot be certain of the effects; we cannot be sure that the circumstances of the witnesses are such as have before given rise
to true testimony, and consequently we cannot be sure, that
the testimony is true." According to this view, we can be
absolutely certain of nothing, the knowledge of which is obtained by testimony: but every man's experience will contradict this statement; for who needs to be informed, that there
are thousands of facts, known no otherwise than by testimony,
of the certainty of which we have no more doubt than of our
own existence. Supposing then the fact which is contrary to
the uniformity of causation, to be attended with testimony of
this kind, the equipoise must exist.
But there is one consideration which seems equally to have
escaped the notice of Mr. Hume and this Essayist. It is, that
the same contrariety of evidence, and consequent equipoise,
destroying all assent, must take place between the evidence of
our senses and the uniformity of causation; for there is no reason why this equipoise, and mutual destruction of conflicting
evidence, should exist in relation to testimony alone: the very
same thing must necessarily occur, if a fact be observed by
our senses, which is contrary to the established course of nature. Thus, if we should see with our own eyes a cubic inch
of ice placed in a temperature of 200 degrees of Fahrenheit,
and should distinctly observe, that it remained unmelted at the
expiration of an hour, we could not believe the fact; for although nothing can be more certain to us than what we see;
yet as this fact implies, that the same causes do not always
produce the same effects; and as this is a self-evident truth,
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