It is excited by the lively impression; and this vivacity is conveyed to the related idea, without any great diminution in the passage, by reason of the smooth transition and the propensity of the imagination.
But suppose, that this propensity arises from some other principle, besides that of relation; it is evident it must still have the same effect, and convey the vivacity from the impression to the idea.

Now this is exactly the present case.

Our memory presents us with a vast number of instances of perceptions perfectly resembling each other, that return at different distances of time, and after considerable interruptions.

This resemblance gives us a propension to consider these interrupted perceptions as the same; and also a propension to connect them by a continued existence, in order to justify this identity, and avoid the contradiction, in which the interrupted appearance of these perceptions seems necessarily to involve us.

Here then we have a propensity to feign the continued existence of all sensible objects; and as this propensity arises from some lively impressions of the memory, it bestows a vivacity on that fiction: or in other words, makes us believe the continued existence of body.