OF THE COMPONENT PARTS OF OUR REASONINGS CONCERNING CAUSE AND EFFECT.
Though the mind in its reasonings from causes or effects, carries its
view beyond those objects which it sees or remembers, it must never
lose sight of them entirely, nor reason merely upon its own ideas,
without some mixture of impressions, or at least of ideas of the
memory, which are equivalent to impressions. When we infer effects from
causes, we must establish the existence of these causes; which we have
only two ways of doing, either by an immediate perception of our memory
or senses, or by an inference from other causes; which causes again we
must ascertain in the same manner, either by a present impression or
by an inference from _their_ causes, and so on, till we arrive at some
object, which we see or remember. 'Tis impossible for us to carry on
our inferences _in infinitum_; and the only thing that can stop them,
is an impression of the memory or senses, beyond which there is no room
for doubt or inquiry.
To give an instance of this, we may choose any point of history, and
consider for what reason we either believe or reject it. Thus, we
believe that Cæsar was killed in the senate-house on the _ides_ of
_March_, and that because this fact is established on the unanimous
testimony of historians, who agree to assign this precise time and
place to that event. Here are certain characters and letters present
either to our memory or senses; which characters we likewise remember
to have been used as the signs of certain ideas: and these ideas
were either in the minds of such as were immediately present at that
action, and received the ideas directly from its existence; or they
were derived from the testimony of others, and that again from another
testimony, by a visible gradation, till we arrive at those who were
eye-witnesses and spectators of the event. 'Tis obvious all this chain
of argument or connexion of causes and effects, is at first founded
on those characters or letters, which are seen or remembered, and
that without the authority either of the memory or senses, our whole
reasoning would be chimerical and without foundation. Every link of
the chain would in that case hang upon another; but there would not be
any thing fixed to one end of it, capable of sustaining the whole; and
consequently there would be no belief nor evidence. And this actually
is the case with all _hypothetical_ arguments, or reasonings upon a
supposition; there being in them neither any present impression, nor
belief of a real existence.
I need not observe, that 'tis no just objection to the present
doctrine, that we can reason upon our past conclusions or principles,
without having recourse to those impressions, from which they first
arose. For even supposing these impressions should be entirely effaced
from the memory, the conviction they produced may still remain; and
'tis equally true, that all reasonings concerning causes and effects
are originally derived from some impression; in the same manner, as
the assurance of a demonstration proceeds always from a comparison of
ideas, though it may continue after the comparison is forgot.