As to those impressions, which arise from the senses, their ultimate cause is, in my opinion, perfectly inexplicable by human reason, and it will always be impossible to decide with certainty, whether they arise immediately from the object, or are produced by the creative power of the mind, or are derived from the author of our being.  Nor is such a question any way material to our present purpose.  We may draw inferences from the coherence of our perceptions, whether they be true or false; whether they represent nature justly, or be mere illusions of the senses.

When we search for the characteristic, which distinguishes the memory from the imagination, we must immediately perceive, that it cannot lie in the simple ideas it presents to us; since both these faculties borrow their simple ideas from the impressions, and can never go beyond these original perceptions.  These faculties are as little distinguished from each other by the arrangement of their complex ideas.  For though it be a peculiar property of the memory to preserve the original order and position of its ideas, while the imagination transposes and changes them, as it pleases; yet this difference is not sufficient to distinguish them in their operation, or make us know the one from the other; it being impossible to recal the past impressions, in order to compare them with our present ideas, and see whether their arrangement be exactly similar.  Since therefore the memory, is known, neither by the order of its complex ideas, nor the nature of its simple ones; it follows, that the difference betwixt it and the imagination lies in its superior force and vivacity.  A man may indulge his fancy in feigning any past scene of adventures; nor would there be any possibility of distinguishing this from a remembrance of a like kind, were not the ideas of the imagination fainter and more obscure.

It frequently happens, that when two men have been engaged in any scene of action, the one shall remember it much better than the other, and shall have all the difficulty in the world to make his companion recollect it.  He runs over several circumstances in vain; mentions the time, the place, the company, what was said, what was done on all sides; till at last he hits on some lucky circumstance, that revives the whole, and gives his friend a perfect memory of every thing.  Here the person that forgets receives at first all the ideas from the discourse of the other, with the same circumstances of time and place; though he considers them as mere fictions of the imagination.  But as soon as the circumstance is mentioned, that touches the memory, the very same ideas now appear in a new light, and have, in a manner, a different feeling from what they had before.  Without any other alteration, beside that of the feeling, they become immediately ideas of the memory, and are assented to.

Since, therefore, the imagination can represent all the same objects that the memory can offer to us, and since those faculties are only distinguished by the different feeling of the ideas they present, it may be proper to consider what is the nature of that feeling.  And here I believe every one will readily agree with me, that the ideas of the memory are more strong and lively than those of the fancy.