In high school economics, students are taught that there are four factors of production: land, labour, capital and enterprise, and that the rewards for each are respectively: rent, wages, capital and profit.
Capital here is defined as capital goods - goods which are used in the production of goods and services, not in the sense of financial capital. Given this, I can't see why the reward for capital would be interest, if capital is defined in this way.
One argument I've heard is that any sort of investment is typically invested into capital goods and the opportunity cost of this investment is some rate of interest. I'm not entirely convinced because you could easily apply the same logic to, say, investment into land.
Does anyone the originator of the concept of the four factors of production and their respective rewards? The Wikipedia article is somewhat informative in that it differentiates between the different forms of capital, but it still doesn't answer my question about the returns from the factors of production.