 a requisite Salmon’s definition of processes and causal transmission does not meet. By way of a counterexample, Hitchcock observed that based on Salmon’s theory a blue spot impressed on a billiard ball by a stick on which a player had put blue chalk would count as a mark transmitted by a causal process, and furthermore one would be unable to tell why the linear momentum of the billiard ball should be included in the explanation of its movement, whereas the blue mark should not.