A fox should not be of the jury at a goose trial.

A person whose interests align with one side should not help decide that side’s case. The image stresses the unfairness of allowing a likely beneficiary to judge those who may be harmed by the decision.

It warns against conflicted judgment rather than merely against cleverness or appetite. Its predator-and-prey image is figurative, not a literal claim about animals; fair decisions require impartial judges or panels, especially where a party is vulnerable to the decision-maker’s preferences, profit, or hostility.

Use it when criticizing a biased appointment to a jury, committee, inquiry, or other decision-making role. It is apt where the appointee has a clear stake in the outcome and their participation would undermine confidence in fairness.