Full Idea
A being cannot naturally go out of existence. For even if a ship or a plank ceases to be a ship or a plank, it never naturally ceases to be a being. For a being, unless it is annihilated, does not cease to be a being. To annihilate is a supernatural task.
Gist of Idea
Only supernatural means could annihilate anything once it had being
Source
Thomas Hobbes (De Mundo (On the World) [1642], 12.5)
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.679
A Reaction
This idea was becoming an orthodoxy in Hobbes's time, and leads to the various conservation laws in physics.