Kevin Hoover suggests that William Nordhaus relied on the labor theory of value in his paper on light (Letters, April 24). Yet Nordhaus actually used knowledge, not labor. He offered a method to measure innovation: the discovery and sharing of valuable new knowledge. Nordhaus measured the amount of knowledge per unit of time and observed that knowledge about light was growing exponentially, surpassing traditional measures of economic development. It is the time price over time that truly deserves our attention. Read More ›