 Reichenbach introduced the mark method in The Philosophy of Space and Time (1928 [1957]) to distinguish causal processes from pseudo-processes. Unlike the latter, causal processes have the capacity to transmit marks, namely various sorts of signals or information. Salmon’s favorite example is that of a rotating spotlight placed at the center of a circular room, which casts a spot of light on the wall. The light beam that travels from the source to the wall is a causal process, whereas the light spot that moves around the wall is a pseudo-process. In order to convince us that the light ray is a genuine process, Salmon invites us to consider what happens if a red filter is placed near the source of the beam: the color of the beam will turn red, and so will the spot on the wall. By interposing a red filter between the source and the beam a mark has been introduced, which is then transmitted along the beam. By contrast, the moving spot on the wall is not a causal process, because it lacks the capacity to transmit information; if for instance someone were to place a piece of red cellophane on the wall at some point, the light would turn red when the beam hit the cellophane, but the color would not be retained as the spot moves on.