 This move was made necessary to obviate a problem connected with a counterexample suggested by Nancy Cartwright. This can be summarized as follows: back to the example of the rotating spotlight, suppose that a few nanoseconds before a piece of red cellophane is placed on the wall and turns the moving spot red someone places a red lens on the rotating beacon, so that the beam remains red, not because of the cellophane placed on the wall, but because of the lens placed near the light source. In Salmon’s words: