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Memory is sensation that is fading, old, and past (and no different essentially from imagination).
The decay of sense in men waking is not the decay of the motion made in sense, but an obscuring of it, in such manner as the light of the sun obscureth the light of the stars; which stars do no less exercise their virtue by which they are visible in the day than in the night. But because amongst many strokes which our eyes, ears, and other organs receive from external bodies, the predominant only is sensible; therefore the light of the sun being predominant, we are not affected with the action of the stars. And any object being removed from our eyes, though the impression it made in us remain, yet other objects more present succeeding, and working on us, the imagination of the past is obscured and made weak, as the voice of a man is in the noise of the day. From whence it followeth that the longer the time is, after the sight or sense of any object, the weaker is the imagination. For the continual change of man's body destroys in time the parts which in sense were moved: so that distance of time, and of place, hath one and the same effect in us. For as at a great distance of place that which we look at appears dim, and without distinction of the smaller parts, and as voices grow weak and inarticulate: so also after great distance of time our imagination of the past is weak; and we lose, for example, of cities we have seen, many particular streets; and of actions, many particular circumstances. This decaying sense, when we would express the thing itself (I mean fancy itself), we call imagination, as I said before. But when we would express the decay, and signify that the sense is fading, old, and past, it is called memory. So that imagination and memory are but one thing, which for diverse considerations hath diverse names.
Hobbes, Lev II 3