William Cowper - A Riddle

I am just two and two, I am warm, I am cold,
And the parent of numbers that cannot be told.
I am lawful, unlawful -- a duty, a fault,
I am often sold dear, good for nothing when bought;
An extraordinary boon, and a matter of course,
And yielded with pleasure when taken by force.

William Cowper

http://www.poemhunter.com/poem/a-riddle-18/