To give an instance of this, we may chuse any point of history, and consider for what reason we either believe or reject it.  Thus we believe 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.

SECT.  V. OF THE IMPRESSIONS OF THE SENSES AND MEMORY.

In this kind of reasoning, then, from causation, we employ materials, which are of a mixed and heterogeneous nature, and which, however connected, are yet essentially different from each other.  All our arguments concerning causes and effects consist both of an impression of the memory or, senses, and of the idea of that existence, which produces the object of the impression, or is produced by it.  Here therefore we have three things to explain, viz.  First, The original impression.  Secondly, The transition to the idea of the connected cause or effect.  Thirdly, The nature and qualities of that idea.