Sir Walter Scott is said to have seen a phantom of the dead Byron.

It is possible that all of us are liable to momentary hallucinations at times of exceptional nervous exhaustion, though they are too fugitive to excite our attention.
When not brought on by exhaustion or artificial means, the hallucinations of the sane have their origin in a preternatural power of imagination.

It is well known that this power can be greatly improved by attention and cultivation.

Goethe used to exercise himself in watching for ocular spectra, and could at will transform these subjective sensations into definite forms, such as flowers; and Johannes Mueller found he had the same power.[59] Stories are told of portrait painters who could summon visual images of their sitters with a vividness equal to that of reality, and serving all the purposes of their art.

Mr.
Galton's interesting inquiries into the power of "visualizing" would appear to prove that many people can at will sport on the confines of the phantom world of hallucination.