The person who borrows has found by experience that he cannot borrow without paying interest and his answer to the question as to why interest is paid is likely to be that that is the only way in which money can be obtained. But the question goes much deeper than this. When the person borrows money and invests it in capital which is used up in production how does it happen that this capital re-creates itself in the product? Not only how does it reproduce itself but why does it produce a surplus in addition to the equivalent of itself, - a surplus which we know as interest? A great many varieties of answers have been given to these questions and in the following paragraphs we shall consider briefly a few of them.