A Riddle Wrapped in a Poem

I try to keep my antennas up for writers mentioned by writers that I like.  In reading Jane Austen’s Sense and Sensibility, the name William Cowper cropped up.  With the typical British obfuscation of such things, his name is pronouced “Cooper”, and he was evidently a forerunner of the Romantic movement (Wordsworth, etc.)  In looking through his poems, I came up with this one, which poses a nifty riddle.  I’ve spent weeks, on and off, in banging my head against it, but still don’t have a solution that satisfies me.  Suggestions welcome … as long as they come from your own noodle, and not The Great Google.

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.

Walter Scott on Jane Austen — and vice versa

From the March 14, 1826 entry in Scott’s journal:

[I] read again, and for the third time at least, Miss Austen’s very finely written novel of Pride and Prejudice. That young lady had a talent for describing the involvements and feelings and characters of ordinary life, which is to me the most wonderful I ever met with. The Big Bow-wow strain I can do myself like any now going; but the exquisite touch, which renders ordinary commonplace things and characters interesting, from the truth of the description and the sentiment, is denied to me. What a pity such a gifted creature died so early!

I mean, this guy — no slouch of a writer himself — had humility.  He also was a pretty fair judge of talent.

Miss Austen also had some nice words for Mr. Scott, barbed though they may be.  From a letter of Sept. 28, 1814 to her niece, Anna Austen:

Walter Scott has no business to write novels, especially good ones. – It is not fair. – He has Fame & Profit enough as a Poet, and should not be taking the bread out of other people’s mouths. – I do not like him, & do not mean to like Waverley if I can help it – but fear I must…