Notes on Hume's Treatise
by G. J. Mattey
PART III. OF KNOWLEDGE AND PROBABILITY
§ IV. Of the component parts of our reasonings concerning cause and effect.
In all our causal reasoning, we must begin with "some object, which we see or remember." The impressions of sense and memory are the only things that can serve to prevent a regress. Without a fixed stopping point, "there wou'd be no belief nor evidence." Hypothetical arguments are lacking in belief and evidence, "there being in them, neither any present impression, nor belief of a real existence."
We need not be able to call up the original impression: the memory of the conviction it produced is enough even if "these impressions shou'd be entirely effac'd from the memory." Causal reasoning proceeds in the same way as a demonstration based on comparison if ideas, which "may continue after the comparison is forgot."