When a man lays his hand firmly on my shoulder, I may be in little doubt whether I feel a sensation or do not; but when he touches my back very lightly, I may easily be in doubt, and may ask myself in perplexity whether I have really been touched or whether I have merely imagined it.

As a vessel recedes and becomes a mere speck upon the horizon, I may well wonder, before I feel sure that it is really quite out of sight, whether I still see the dim little point, or whether I merely imagine that I see it.
On the other hand, things merely imagined may sometimes be very vivid and insistent.

To some persons, what exists in the imagination is dim and indefinite in the extreme.

Others imagine things vividly, and can describe what is present only to the imagination almost as though it were something seen.

Finally, we know that an image may become so vivid and insistent as to be mistaken for an external thing.