  Modern approaches to causal modeling give a central role to interventions,
which require the active input of an observer and introduces an explicit
`causal arrow of time'. Causal models typically adopt a mechanistic
interpretation, according to which the direction of the causal arrow is
intrinsic to the process being studied. Here we investigate whether the
direction of the causal arrow might be a contribution from the observer, rather
than an intrinsic property of the process. Working within a counterfactual and
non-mechanistic interpretation of causal modeling developed in
arXiv:1806.00895, we propose a definition of a `quantum observational scheme'
that we argue characterizes the observer-invariant properties of a causal
model. By restricting to quantum processes that preserve the maximally mixed
state (unbiasedness) we find that the statistics is symmetric under reversal of
the time-ordering. The resulting model can therefore accommodate the idea that
the causal arrow is observer-dependent, indicating a route towards reconciling
the causal arrow with time-symmetric laws of physics.
