The courts. See the link. In practice, this definition has rarely [1] been a problem. There are more serious concerns. For example, a real problem with posses

The courts. See the link. In practice, this definition has rarely [1] been a problem. There are more serious concerns. For example, a real problem with possession-based laws (as opposed to distribution-based laws) is how comparatively easy it is to frame innocent people [2]. Law enforcement likes possession-based laws, because they are an order of magnitude easier to prove and prosecute. [1] Which is not to say "never". [2] http://www.bbc.co.uk/news/uk-england-london-11397515 http://www.bbc.co.uk/news/uk-england-london-11397515