 In order to establish a nomic, intrinsic distinction between forward causal processes and backward causal processes one has to take departure in four assumptions. (i) Process tokens and process types are distinct in the sense that only process types are reversible, process tokens are not. (ii) A normal observer will describe causal processes propagating forward in time in terms of positive mass and positive energy states pointing into her future whereas she will describe the same tokens in terms of negative mass and energy states pointing into her past. This reflects two possible solutions of the four-momentum vector in the theory of relativity. (iii)Thus, one must distinguish between a passive time reversal operation and an active time reversal operation. The passive transformation is applied to the same process token by describing it in terms of opposite coordinates and opposite energy states. The active transformation, in contrast, brings about another token of the same process type in virtue of some physical translation or rotation of the system itself, both tokens having the same energy sign pointing in the same direction of time. (iv) The description in terms of positive mass and the positive energy flow corresponds to the intrinsic order of the propagation.