For a long time, Marshall's chief contribution to economic science was considered to be Book V of Principles of Economics, which is the “analytic core” of his thought. Book V, “General Relations of Demand, Supply and Value,” deals with the important theme of economic equilibrium that ipso facto rules out any possibility of coping with economic development. When Alfred Marshall describes Book V, he points out that “it is not descriptive, nor does it deal constructively with real problems. But it sets out the theoretical backbone of our knowledge of the causes that govern value” (1961, vol. 1, p. 324); in short, it “deals with abstractions” (1898, p. 52). The existence of Book V has been supposed by some scholars to be sound enough proof that Alfred Marshall was not interested in the question of economic development. More recently, however, it has been suggested that one of the main Marshallian concerns was economic development,—“the high theme of economic progress,” as he called it (Marshall 1961, vol. 1, p. 461).