Ed Feser on anomalous monism

Writing here on Donald Davidson's anomalous monism,  Ed agrees that there are no strict laws connecting mental events with brain events. "The 'anomalous' half of anomalous monism is thus well-established,"  he says.  But he does not accept that mental events interact with physical events via efficient causation:

As I have argued elsewhere (e.g. here), the correct way to understand mental-physical “interaction” is on the model of what Aristotelians call formal causation rather than efficient causation.

How are we to understand this formal causation? Not on the model of a statue's shape to its matter, for this correspondence is strict.  Nor, surely, on the model of a building's plan to its finished state, for this is under-determined, requiring an agent to 'fill out the details.'  How then?