While there may be a certain surface plausibility to Hume's account of memory, its inadequacies quickly come to light.  First, as Hume himself recognizes, there is nothing in principle to prevent an idea of memory from being decidedly faint and without force (just as ideas of imagination can be vivid and forceful).  Hume can and does say that when memory ideas are faint, they are often not recognized as memory images, but are instead taken to be of the imagination.  But this response confuses the issue.  Our concern here, and the one that Hume is addressing in the relevant sections of the Treatise, is with the metaphysics of memory and not with its epistemology.  So it doesn't matter that sometimes memory images may be mistaken for images of the imagination.  For even in describing such a circumstance, one assumes that the fact of the matter is that the image in question is a memory image and that the subject has made a mistake in judging the image to be of imagination.  We are trying to figure out the criteria according to which an image is of memory and not the conditions under which one can introspectively know that this.