Some other translations, including the KJV use the term "replenish rather than "multiply."
Why would a planet void, cold and dark, having no atmosphere or sun
need to be replenished (abe: with life)?
Replenish comes from the French,
replenir itself from the latin
plenus - which means 'full'. So four hundred years ago - it probably didn't mean 'to fill up again'. Which is presumably why your more modern translations don't use that word.
abe: the prefix 're' can mean 'again or backwards' as in
retard,
revolve,
reallign but in old French it can be used as an intensive prefix (makes the following word have the same meaning but moreso). As in mankind was instructed not to just fill the world, but
really fill the world.
We see this prefix use in
redolent (very smelly)
research (to search in depth)
resplendant (very brilliant)
religion (very binding (ligio - as in ligaments))
regret (gret meaning 'to weep')
etc
Edited by Modulous, : No reason given.
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