Identity has to do with "place". In less mobile societies, the place where a person finds him/herself tells us something about his/her identity. In pre-industrial times, gatekeepers had the function to control access of people to particular places, i.e. the gatekeepers function was to identify people and then decide whether somebody's identity would allow that person to physically occupy another place - a town, a building, a vehicle, etc.
In modern societies, the unambiguous nature of place has been weakened. There is a great amount of physical mobility, and ever since the emergence and spread of electronic communication technologies there has been a "virtualisation" of places in what today we call "virtual space" (unlike place, space has been a virtual reality from the beginning, a mathematical formula) The question as to who one is no longer coupled to the physical abode. Highly mobile and virtualised social contexts require a new generation of gatekeepers which biometric technology aims to provide.
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The water-clocks are an early long-distance-communication-system. Every communicating party had exactly the same jar, with a same-size-hole that was closed and the same amount of water in it. In the jar was a stick with different messages written on. When one party wanted to tell something to the other it made a fire-sign. When the other answered, both of them opened the hole at the same time. And with the help of another fire-sign closed it again at the same time, too. In the end the water covered the stick until the point of the wanted message.