In terminology, existent, as compared to a non-existent, is a thing having reality or existence. [1]

The following are related quotes:

“A similar inquiry is made by those who inquire into the ‘number of existents’; for they inquire whether the ultimate constituents of ‘existing things’ are on or many, and if many whether a finite or an infinite plurality. So they are inquiring whether the principle or element is one or many.”
Aristotle (350BC), Physics (§1.2:315-16) [2]

“The law of sufficient reason, applied to actual existents, reduces itself to the assertion of final causes.”
Bertrand Russell (c.1920), volume on Gottfried Leibniz; cited by Arthur Lovejoy (1933) in The Great Chain of Being (pg. 146) [2]

See also
Reaction existence
Cease to exist

1. (a) Existent – Merriam-Webster.com.
(b) Google Dictionary (2019).
2. Lovejoy, Arthur. (1933). The Great Chain of Being: a Study of the History of an Idea. Harvard University Press, 1936.
2. Aristotle. (322BC). The Complete Works of Aristotle, Volume One (editor: Jonathan Barnes). Princeton University Press, 1995.

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