One: Some single thing; what a
single thing counts as. The term "one" has mostly a counting
sense, and the number 1 or numerical
concept 'one' is one of the few numbers all men always and everywhere,
in all languages so far as is known, have recognized. (Some human
civilizations had not much use for more than "one", "two", "three". And
indeed a few people can recognize precisely five things at a glance, but
most can distinguish one, two or three things at a glance.)
It is somewhat interesting that the numerical concept
one can be defined or closely
approximated by a formula that only
contains logical terms including
identity: In case any
predicate F satisfies this formula
(Ex)(Fx & (y)(Fy > x=y)) it is true of precisely one thing 
for the formula says there is some thing that is F and every thing that
is F is the same as that thing.
