And this brings me to the most important Consideration in Political Oeconomies, viz. how to make a Par and Equation between Lands and Labour, so as to express the Value of any thing by either || alone. To which purpose, suppose two Acres of Pasture-land inclosed, and put thereinto a wean’d Calf, which I suppose in twelve Months will become 1 C. heavier in eatable Flesh; then 1 C. weight of such Flesh, which suppose fifty days Food, and the Interest of the Value of the Calf, is the value or years Rent of the Land. But if a mans labour ——– for a year can make the said Land to yield more than sixty days Food of the same, or of any other kind, then that overplus of days food is the Wages of the Man ; both being expressed by the number of days food. That some Men will eat more than others, is not material, since by a days food we understand 1/100 part of what 100 of all Sorts and Sizes will eat, so as to Live, Labour, and Generate. And that a days food of one sort, may require more labour to produce, than another sort, is not material, since we understand the easiest-gotten food of the respective Countries of the World. […] the days food an an adult Man, at a Medium, and not the days labour, is the common measure of Value, and seems to be as regular and constant as the value of fine Silver. […]  Wherefore I valued an Irish Cabbin at the number of days food, which the Maker spent in building of it.
By the same way we must make a Par and Equation between Art and Simple Labour […]
William Petty, The Political Anatomy of Ireland, in The Economic Writings of Sir William Petty vol. I, ed. Charles Henry Hull (Routledge/Thoemmes 1997)