According to vulgar apprehension, memory is an immediate knowledge of something past.  Our author [i.e., Hume] does not admit that there is any such knowledge in the human mind.  He maintains that memory is nothing but a present idea or impression. But, in defining what he takes memory to be, he takes for granted that kind of memory which he rejects.  For can we find by experience, that an impression, after its first appearance to the mind, makes a second, and a third, with different degrees of strength and vivacity, if we have not so distinct a remembrance of its first appearance, as enables us to know it, upon its second and third, notwithstanding that, in the interval, it has undergone a very considerable change. 