Cost-benefit analysis (which puts a discount rate on the future, and measures in dollars and cents) is wicked.

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One is that no matter how sophisticated the portfolio analysis used, it is simply not possible to discount mathematically the chances that Damien Hirst's polka dots are going to appeal more in 20 years than Jeff Koons's dinner plates.

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In the last analysis, however, the issue is more basic than discount coupons.

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