Even in our minds the two things are often confounded.
We often have to pause and think in order to decide whether a mental perception of which we are conscious is a remembrance of a reality, or a revival of some image formed at some previous time, perhaps remote, by a vivid description which we have read or heard, or even by our own fancy.
"Is that really so, or did I dream it ?" How often is such a question heard.
And persons have been known to certify honestly, in courts of justice, to facts which they think they personally witnessed, but which were really pictured in their minds in other ways.

The picture was so distinct and vivid that they lost, in time, the power of distinguishing it from other and, perhaps, similar pictures which had been made by their witnessing the corresponding realities.
Indeed, instead of being surprised that these different origins of present mental images are sometimes confounded, it is actually wonderful that they can generally be so clearly distinguished; and we can not explain, even to ourselves, what the difference is by which we do distinguish them.
For example, we can call up to our minds the picture of a house burning and a fireman going up by a ladder to rescue some person appearing at the window.

Now the image, in such a case, may have had several different modes of origin.1.We may have actually witnessed such a scene the evening before.2.Some one may have given us a vivid description of it.3.We may have fancied it in writing a tale, and 4.

We may have dreamed it.

Here are four different prototypes of a picture which is now renewed, and there is something in the present copy which enables us, in most cases, to determine at once what the real prototype was.