Section IV. Of the component parts of our reasonings concerning cause and effect.
THO' 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. It is 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 enquiry.
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 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, 'till we arrive at those who were eyewitnesses and spectators of the event. 'Tis obvious all this chain of argument or connection 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 '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, tho' it may continue after the comparison is forgot.