以下来自Wikipedia——经济学中的利率：

In economics, the rate of interest is the price of credit, and it plays the role of the cost of capital. In a free market economy, interest rates are subject to the law of supply and demand of the money supply, and one explanation of the tendency of interest rates to be generally greater than zero is the scarcity of loanable funds.
Over centuries, various schools of thought have developed explanations of interest and interest rates. The School of Salamanca justified paying interest in terms of the benefit to the borrower, and interest received by the lender in terms of a premium for the risk of default.[citation needed] In the sixteenth century, Martín de Azpilcueta applied a time preference argument: it is preferable to receive a given good now rather than in the future. Accordingly, interest is compensation for the time the lender forgoes the benefit of spending the money.
On the question of why interest rates are normally greater than zero, in 1770, French economist Anne-Robert-Jacques Turgot, Baron de Laune proposed the theory of fructification. By applying an opportunity cost argument, comparing the loan rate with the rate of return on agricultural land, and a mathematical argument, applying the formula for the value of a perpetuity to a plantation, he argued that the land value would rise without limit, as the interest rate approached zero. For the land value to remain positive and finite keeps the interest rate above zero.
Adam Smith, Carl Menger, and Frédéric Bastiat also propounded theories of interest rates.[8] In the late 19th century, Swedish economist Knut Wicksell in his 1898 Interest and Prices elaborated a comprehensive theory of economic crises based upon a distinction between natural and nominal interest rates. In the 1930s, Wicksell's approach was refined by Bertil Ohlin and Dennis Robertson and became known as the loanable funds theory. Other notable interest rate theories of the period are those of Irving Fisher and John Maynard Keynes.

马克思的利息的“剩余价值”说正是关于利息的“垃圾”学说中的一种。利润、利息、租金，它们作为“剩余价值”的一部分，毫无疑问是需要被废除的。马克思有这样的结论，也并不令人意外，这是从斯密-李嘉图-马克思的错误的“劳动价值论”所得出的必然的——也必然错误——的结论。

你既不理解“偏好”的含义，也不理解什么是“时间偏好”。但有一点可以确定，就是你自己的行为也在践行着“时间偏好”所描述的事实。

例如你今天赚了100元，为什么要拿出譬如50元吃喝穿用，而不一直储蓄？？如果不存在时间偏好，就像你所想的，利息为0，那么，这100元的价值，在今天、明天、将来都一样，换言之，你合理的行为结果将是：永不将这100元消费，而无休止的储蓄下去。事实上，你做不到这一点，这也就证明了：时间偏好的正确性。

即使你铁嘴钢牙，宣称利息是勒索是敲诈是不道德，需要废除，但嘴上如此，身体却会出卖你。

利息的存在是个任何人无法否认的事实。你干嚎也没用，道德谴责也没用。你嘴再硬，也拿它没辙。因为事实就是如此。