投资往往为被描述为这样一个过程，现在投入一些钱，希望未来能收回更多的钱。在伯克希尔哈撒韦（Berkshire Hathaway），我们采用更高的标准，将投资定义为“现在将购买力让渡给他人，合理期待未来支付明义收益税率后，仍能获得更高的购买力”。简言之，投资就是放弃现在的消费，以便将来有能力消费更多。

    从我们的定义可以得出一个重要结论：衡量投资风险高低的指标不应是贝塔值（一个基于波动率的华尔街术语，常用于衡量风险），而是持有期满后投资人出现购买力损失的（合理）概率。资产价格可能大幅波动，但只要有理由肯定投资期满后它们带来的购买力能得到提升，这项投资就没有风险。稍后我们还会看到，价格没有波动的资产也可能充满风险。

    投资选择林林总总，但大体可分为三类，理解每一类的特点很重要。下面，我们将展开详细的分析。

    市场上以特定货币结算的投资包括货币市场基金、债券、按揭、银行存款和其他工具。大多数此类投资都被视为“安全”，但事实上却是可能属于最危险的资产。它们的贝塔值可能是零，但风险巨大。

    上个世纪，这类投资工具摧毁了很多国家投资者的购买力，尽管投资者一直能够按时收到支付的本息。而且，这样的糟糕结果将来还会不断重现。政府决定货币的最终价值，而系统性力量有时会推动它们制定导致通胀的政策。这些政策一不留神就会失控。

    Investing is often described as the process of laying out money now in the expectation of receiving more money in the future. At Berkshire Hathaway (BRKA) we take a more demanding approach, defining investing as the transfer to others of purchasing power now with the reasoned expectation of receiving more purchasing power -- after taxes have been paid on nominal gains -- in the future. More succinctly, investing is forgoing consumption now in order to have the ability to consume more at a later date.

    From our definition there flows an important corollary: The riskiness of an investment is not measured by beta (a Wall Street term encompassing volatility and often used in measuring risk) but rather by the probability -- the reasoned probability -- of that investment causing its owner a loss of purchasing power over his contemplated holding period. Assets can fluctuate greatly in price and not be risky as long as they are reasonably certain to deliver increased purchasing power over their holding period. And as we will see, a nonfluctuating asset can be laden with risk.

    Investment possibilities are both many and varied. There are three major categories, however, and it's important to understand the characteristics of each. So let's survey the field.

    Investments that are denominated in a given currency include money-market funds, bonds, mortgages, bank deposits, and other instruments. Most of these currency-based investments are thought of as "safe." In truth they are among the most dangerous of assets. Their beta may be zero, but their risk is huge.

    Over the past century these instruments have destroyed the purchasing power of investors in many countries, even as these holders continued to receive timely payments of interest and principal. This ugly result, moreover, will forever recur. Governments determine the ultimate value of money, and systemic forces will sometimes cause them to gravitate to policies that produce inflation. From time to time such policies spin out of control.