The idea that there is a clear distinction between typical factors of production and land (and other things "fixed in supply") is not uncontroversial. An interesting article examining the issue is: Florenz Plassmann and T. Nicolaus Tideman Frank Knight's Proposal to End Distinctions among Factors of Production and His Objection to the Single Tax History of Political Economy, FALL 2004; 36: 505 - 519. Jeff Biddle