Robinson has been living on the island for thirty years and his thinking is still dominated by the teaching of Marx. These are the words of Marx:
"The change of value of money that converts it into capital cannot be derived from the money itself, since money in its function of medium of payment does no more than pay the price of the commodity it purchases, and, as hard cash it is value petrified, never varying. Just as little can the change occur in the second act of circulation, the re-sale of the commodity. [For in both cases] equivalents are exchanged, and the commodity is paid for at its full value. We are therefore forced to the conclusion that the change originates in the use-value of the commodity, after its purchase and before its sale." (Marx, Capital I. VI).