He published in 1662 and 1685 a "Treatise of Taxes and
Contributions, the same being frequently to the present state and
affairs of Ireland," of which his view started from the general
opinion that men should contribute to the public charge according to
their interest in the public peace--that is, according to their
riches. "Now, he said, "there are two sorts of riches--one actual,
and the other potential. A man is actually and truly rich according
to what he eateth, drinketh, weareth, or in any other way really and
actually enjoyeth. Others are but potentially and imaginatively
rich, who though they have power over much, make little use of it,
these being rather stewards and exchangers for the other sort than
owners for themselves." He then showed how he considered that
"every man ought to contribute according to what he taketh to
himself, and actually enjoyeth."

In 1674 Sir William Petty published a paper on "Duplicate
Proportion," and in 1679 he published in Latin a "Colloquy of David
with his Own Soul." In 1682 he published a tract called
"Quantulumcunque, concerning Money;" and "England's Guide to
Industry," in 1686. From 1682 to 1687, the year of his death, Sir
William Petty was drawing great attention to the "Essays on
Political Arithmetic," which are here reprinted. There was the
little "Essay in Political Arithmetic, concerning the People,
Housings, Hospitals of London and Paris;" published in 1682, again
in French in 1686, and again in English in 1687. There was the
little "Essay concerning the Multiplication of Mankind, together
with an Essay on the Growth of London," published in 1682, and again
in 1683 and 1686. There was in 1683, "Another Essay in Political
Arithmetic concerning the growth of the City of London." There were