The Welfare Theorems

Pareto is perhaps the leading marginalist. Although he preceded the New Welfare economist by decades, his approach to welfare pointed the way for his successors thirty years later. He wrote:

We have taken this thing called pleasure, value in use, economic utility, ophelimity, to be a quantity; but a demonstration of this has not been given. Assuming this demonstration accomplished, how would this quantity be measured? It is an error to believe we could in general deduce the value of ophelimity from the law of supply and demand. … Hereafter, when we speak of ophelimity it must always be understood that we simply mean one of the systems of indices of ophelimity.
Manual of Political Economy p.112