C.28.MR:2 - Problem

An observed value tells us something about how the existing mechanisms operated. An intervention changes a mechanism. Conditioning on the observed value can therefore answer a different question from imposing that value.

A second difficulty appears when a mathematical or software operation has an unclear physical interpretation. Assigning a value to an indicator, replacing a controller and forcing a physical quantity can change different mechanisms. Reusing the same variable name hides that difference.

Even a clearly specified replacement can change the solvability of a model or its subsequent dynamics. The useful result requires both the model transformation and a derivation for the question actually asked.