Bob revisits capital and interest theory to show why the textbook result “interest = MPK” only holds in a one-good world, and why in actual markets the interest rate emerges from time, prices, and capital valuation—not raw productivity.

• Bob and Alberto Bisin Discuss the Use of Mathematics in Economics: Mises.org/HAP527a
• Bob's Dissertation, "Unanticipated Intertemporal Change in Theories of Interest": Mises.org/HAP527b

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