 Memory-images and imagination-images do not differ in their intrinsic qualities, so far as we can discover.  They differ by the fact that the images that constitute memories, unlike those that constitute imagination, are accompanied by a feeling of belief which may be expressed in the words “this happened.” The mere occurrence of images, without this feeling of belief constitutes imagination; it is the element of belief that is distinctive in memory. [Russell 1921, pages 175-176]