Numismatics presents a problem so profound that it is ignored by most coin collectors: what to collect. Coins have been issued for well over 3000 years by thousands, if not tens of thousands, of political entities. This has presented a problem for coin collectors that we stamp people have not yet had to face. There are not catalogs that list all the coins that have ever been issued as we have in stamps. And how could there be? There are too many coins from too many places that are no longer self governing. And many coin issues are undocumented, issued long before agencies of the state formally ordered such things. Such coins were issued by whim, sometimes with little state oversight and new varieties of ancient and medieval coinage are constantly being discovered. Stamps have only been around for 170 years, all in the literate period of world history, where researchers have been able to document issues, know press runs and list varieties almost as they were issued. In the last fifty years there has been no unknown stamp issue discovered of any  classic stamp. Virtually unlimited, uncataloged varieties has been what has made coin collecting so difficult for numismatists whereas philately's defined limits have allowed stamp collectors to be more expansive in what they collect. This is what accounts for modern American numismatics narrow interest in only a small number of United States coins in third party graded quality. It would be just too complicated a hobby otherwise.