that Caesar 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, it will we arrive at those who were eyewitnesses and spectators
of the event. It is 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 it is 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 it is 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.