"Sleep suspends many of the faculties of the vital and intellectual principle—drunkenness and disease will either temporarily or permanently derange them. Madness, or idiotcy, may utterly extinguish the most excellent and delicate of these powers. In old age the mind gradually withers; and as it grew and strengthened with the body, so does it with the body sink into decrepitude."
He also considered that:
"It is probable that what we call thought is not an actual being, but no more than the relation between certain parts of that infinitely varied mass, of which the rest of the universe is composed, and which ceases to exist so soon as those parts change their position with regard to each other. Thus color, and sound, and taste, and odor, exist only relatively."
Even granted that mind or thought be a part of, or in fact, the soul, then he asks in what manner it could be made a proof of its imperishability, as all that we see or know perishes and is changed.
Here then comes the query, "Have we existed before birth?" A difficult possibility to conceive of individual intelligence and if unprovable against the theory of existence after death.
He then winds up the whole by thinking that it is impossible that,
"we should continue to exist after death in some mode totally inconceivable to us at present."
and that only those who desire to be persuaded are persuaded.
This is but a rough outline of some of the principal features of his considerations on soul immortality from a logical basis, and which, after all, only constitute an argument, to which, and the thoughts presented therein, he did not necessarily bind himself. There can be little doubt, independently of what I have quoted, that he did not believe in a future state as popularly accepted. Trelawney asked him on one occasion: "Do you believe in the immortality of the spirit?" Shelley's answer was unmistakable, "Certainly not; how can I? We know nothing; we have no evidence."[B]