On the Relation between the Psychological and Thermodynamic Arrows of Time

In this paper we lay out an argument that generically the psychological arrow of time should align with the thermodynamic arrow of time where that arrow is well-defined. This argument applies to any physical system that can act as a memory, in the sense of preserving a record of the state of some other system. This result follows from two principles: the robustness of the thermodynamic arrow of time to small perturbations in the state, and the principle that a memory should not have to be fine-tuned to match the state of the system being recorded. This argument applies even if the memory system itself is completely reversible and non-dissipative. We make the argument with a paradigmatic system, then formulate it more broadly for any system that can be considered a memory. We illustrate these principles for a few other example systems, and compare our criteria to earlier treatments of this problem.