Definition of POSTERITY
1: the offspring of one progenitor to the furthest generation
2: all future generations
Or!
http://dictionaries.cambridge.org/define.asp?key=posterity*1+0&dict=A
posterity
the people who will exist in the future
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That's what I mean.
The word doesn't exclusively connotate the direct descendants of the speaker.
To say that the context you cite was a comment on birthright citizenship strains the bounds of credulity to a rather large degree...
Hmmm... That ended pretty well.
No, not by itself, but in the Constitution, it says "OURselves and OUR posterity." The context is not generic, particularly since the first part of the preamble is WE the people. They're talking about those who are citizens and THEIR children or descendants. Others can become citizens by naturalization, which is why the Constitution gives the power of naturalization to Congress. Considering this was a fledgling country breaking away from another country, it's ridiculous to presume they were simply talking about "people who will exist in the future" with no ties of loyalty to the country.