Viewpoints were used to implement natural deduction (Fitch in Ether. In order to prove a goal of the form in a viewpoint , it is sufficient to create a new viewpoint that inherits from , assert in , and then prove in . An idea like this was originally introduced into programming language proving by Rulifson, Derksen, and Waldinger [1973 except since Ether is concurrent rather than being sequential it does not rely on being in a single viewpoint that can be sequentially pushed and popped to move to other viewpoints.