Why do we say "Nothing is certain but death and taxes"?

Background:

This proverb appears to have its root in Daniel Defoe's Political History of the Devil (1726):

"Things as certain as death and taxes, can be more firmly believed."

The first reference found in the USA is in a letter Benjamin Franklin wrote to Frenchman Jean-Baptiste Leroy dated November 13, 1789:

"Our new Constitution is now established, and has an appearance that promises permanency; but in this world nothing can be said to be certain, except death and taxes."

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