Thank you, Mason, for your mentioning of importance of the reuse of old sites. One important byproduct of this discussion of creative destruction is the logical conclusion that we have absolutely no theoretical concept of depreciation and that the rules of thumb that we have may be useful but they are far from precise. Of course, if depreciation is unknown because of creative destruction, so too is profit. Future depreciation becomes as much a part of animal spirits as expectations of future demand. Finally, the discovery of new uses of old capital stock introduces the possibility of negative depreciation -- that capital may become increasingly valuable over time. I am convinced that replacement investment represents one of the many soft underbellies of economic theory. Here is a brief extract from my book, Keynes, Investment Theory and the Economic Slowdown: The Role of Replacement Investment and q Ratios (NY and London: St. Martin's and Macmillan, 1989): "A survey of plants with more than 100,000 sq. ft. in the North Central US, which closed between 1 July 1977 and 1 September 1980, found that about half the idle plants had been constructed after 1947 (Institute of Science and Technology, 1984, pp. 6-7). Of the 56 plant closings analysed in the study, more than one-third had been demolished or abandoned or were still on the market. The fact that many of the plants studied in the survey stood idle or even no longer existed did not demonstrate the absolute absence of an economic potential; instead it was a bleak testimony to a failure to recognize that potential. A detailed architectural study of such buildings, combined with relevant economic analysis, found that such buildings might be ideal for certain purposes; for example, the same plants that might be inappropriate for an assembly line process, might well suit the gravity feed and batch milling of pharmaceutical powders (Institute of Science and Technology, 1984, p. 19). In fact, with enough ingenuity such plants can successfully accommodate traditional smokestack industries. For instance, General Motors profitably renovated its half-century old Baltimore manufacturing plant with a 1500 person retrofit team using a winding roller-coaster-like assembly line (Zaslow, 1985, p. 9c). Finally, many old industrial plants near Boston now house some of the prestigious high technology companies springing up in the region." Michael Perelman