Salmon, in Causality and Explanation, suggests that causal processes are demarcated from pseudo-processes by their ability to transmit marks – causal processes can transmit marks; pseudo-processes can’t. About mark transmission: “A mark that has been introduced into a process by means of a single intervention at a point A is transmitted to point B if and only if it occurs at B and at all stages of the process between A and B without additional interventions.” (CaE, p. 197)
Here’s a simple example: There’s a rotating spotlight in the center of a circular room which casts a spot of light on the wall. The light ray traveling from the spotlight to the wall is a causal process; interpose a red filter in the beam near its source and the spot on the wall will be red. The spot of light moving around the wall is a pseudo-process; no interposing of a red filter (or intervention of any sort) can make the spot maintain its redness (or retain a mark of any sort) as it moves on.
We can ask this question, though: How does the process make the mark appear elsewhere within it? (CaE, p. 197) Salmon thinks the answer is ‘astonishingly simple’: it doesn’t (not in any deep sense, anyway). The transmission of a mark from point A in a causal process to point B in the same process just is the fact that it appears at each point between A and B without further interactions.(CaE, p. 197)
I don’t think this is right. It doesn’t get mark transmission right in close by possible worlds (maybe even in our world). Consider a world w. In w there’s a particle a. a can have properties P, Q, and R. Choose any time t. The probability that a will be P at t is 1/3. Likewise for Q and R. There’s another particle b in w. If b strikes a, a will be P, but only during the strike. Suppose b strikes a at t1. b then ricochets and barrells into space. a, then, is P at t1. Suppose also that, by chance, a is P at t2 and at all times between t1 and t2. It seems that, by Salmon’s criteria, a mark (i.e. P) is transmitted from t1 to t2 along the a’s-travels-process. But clearly it’s not. Mark transmission seems to not be as simple as Salmon takes it to be.