PHY.9:4.2 - Choose an effect that carries the distinction into a response

Find a physical interaction through which the sought difference changes a state, transition, force, flow or other observable response. Derive that dependence far enough to see whether the proposed readout retains it. PHY.4 can constrain an unknown law; PHY.6 or PHY.7 can construct the relevant physical evolution.

Compare a small number of plausible arrangements. For each, ask what differs in the predicted readout between the selected physical possibilities under the same preparation. Reject an insensitive arrangement for this question even if it uses a well-established effect. The falling-body case in :5.1 shows why the occurrence of a quantity in one equation is insufficient.

Identify what supplies a reference. A known imposed quantity, a second preparation or a stable response can make the comparison interpretable. State the physical relation that allows the reference to be transported into this measurement. Numerical labels on two devices do not establish that relation.

Retain an unknown coupling as unknown. An available bound or a calibration can be enough; a proposed response without grounds for its coupling remains conditional. Select new characterization only when it can change the intended use.