Measuring an unknown datum so that later you know that datum is time asymmetric because your of how your state of knowledge changes. Defining QM in terms of "m

Measuring an unknown datum so that later you know that datum is time asymmetric because your of how your state of knowledge changes. Defining QM in terms of "measurement" automatically imposes the psychological arrow of time into it. But physics can also be thought of as rules of the from "the joint probabilty distribution of preparations S_j and measurment results R_k is ..." This in turn will depend on prior-probabilities for the S's and R's, and also the rule which tells us P_jk \propto |<S_j|U|R_k>|^2 (where U is some unitary). Notice that this rule treats R's and S's symmetrically. Now the S's and R's are different. The prior for S can have any probability distribution we can engineer, and can even be deterministic. But the prior for the R's must be completely uniform unless we post-select results. So there is a real asymmetry, but it exists in the past and future boundary conditions that we work with, and not in the mechanics (quantum or classical) of what happens in between.