Simon's own explanation for this development is that the very notion of known reserves is deeply flawed,[39] as it does not take into account changes in mining profitability. As richer mines are exhausted, developers turn their attention to poorer sources of the element and eventually develop cheap methods of extracting it, raising known reserves. Thus, for example, copper was so abundant 5000 years ago, occurring in pure form as well as in highly concentrated copper ores, that prehistoric peoples were able to collect and process it with very basic technology.