Although the mind in its reasonings from causes or effects carries its view beyond the objects that it sees or remembers, it must never lose sight of them entirely; it mustn’t reason merely on 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 the causes; which we have only two ways of doing. We can do it either by •an immediate perception of our memory or senses, or by •an inference from other causes; but then we must ascertain the existence of these in the same way, either by a present impression ·or memory· or by an inference from their causes, and so on ·backwards· until we arrive at some object that we see or remember. We can’t carry on our inferences ad infinitum; and the only thing that can stop them is an impression of the memory or senses. Beyond that there is no room for doubt or enquiry.