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Through witnessing each other, in mediated and in natural presence, people construct shared realities. Witnessing in natural presence and witnessing in mediated presence have different effects. Witnessing in natural presence changes the situation because the witness can also decide to act on his or her behalf. Also, the witness can change the nature of an action by testifying about it. For an act to exist in natural presence it has to be witnessed because the act itself elapses. Being seen, having certain interests or shared feelings recognized (without the social judgment and/or limitations that may be part of natural presence) is a powerful trigger for contributing to mediated environments. In mediated presence, which can be endlessly stored and copied by the digital technologies, acts do not have to disappear, which diminishes the need to testify.