 It has also been argued that even when considering time-reversal invariance the argument applies only to deterministic theories, since theories with non-trivial probabilistic state-transition laws are inherently time-asymmetric. As Satosi Watanabe (1965) shows, there can be no genuinely probabilistic theory with both non-trivial forward and backward transition probabilities (see also Healey 1983; Callender 2000). Thus, if quantum mechanics is understood as a fundamentally probabilistic theory, the argument’s scope is limited to what by the argument defenders’ own lights are the less fundamental deterministic theories of classical physics.