“Nevertheless, life and death are mysterious states, and we know little of the resources of either.”
Source: Carmilla
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J. Sheridan Le Fanu 13
Irish writer of Gothic tales and mystery novels 1814–1873Related quotes

“Life, not death, is the great mystery you must confront.”
Source: The Final Testament of the Holy Bible

(zh-TW) 孫子曰:國之上下,死生之地,存亡之道,不可不察也。
The Art of War, Chapter 1 · Detail Assessment and Planning
“Ask yourself if there is any explanation of the mystery of your own life and death.”
The Haunted Hotel: A Mystery of Modern Venice [Rose-Belford, 1878] ( p.288 https://books.google.com/books?id=zyFjcZUO3CUC&pg=PA288)
Also in The Supernatural And English fiction by Glen Cavaliero [Oxford University Press, 1995, ISBN 0-192-12607-5] (p. 39)

“After death the sensation is either pleasant or there is none at all. But this should be thought on from our youth up, so that we may be indifferent to death, and without this thought no one can be in a tranquil state of mind. For it is certain that we must die, and, for aught we know, this very day. Therefore, since death threatens every hour, how can he who fears it have any steadfastness of soul?”
Post mortem quidem sensus aut optandus aut nullus est. Sed hoc meditatum ab adulescentia debet esse mortem ut neglegamus, sine qua meditatione tranquillo animo esse nemo potest. Moriendum enim certe est, et incertum an hoc ipso die. Mortem igitur omnibus horis impendentem timens qui poterit animo consistere?
section 74 http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A2007.01.0039%3Asection%3D74
Cato Maior de Senectute – On Old Age (44 BC)