[lmm]

Not necessarily that we can measure, but that we can reason about. We can't directly measure energy for example, but we know/assume that it obeys certain laws regarding conservation and locality.

If the cat's soul changed the usual laws of physics around the cat, it would destroy our ability to do physics when cats were nearby. It's not about whether we can measure it but about whether we can isolate its effects. If you can't do an experiment that doesn't depend on the state of the entire universe, you can't do an experiment.