No occupation should be subject to a licensing requirement unless incompetence in the performance of tasks normally carried out within that occupation poses a plausible threat to human health or safety.
Principle
unless (if-not)
If incompetence performing job X job X does not pose a → shouldn't be plausible threat to human subject to health or safety license req.
Evaluate
A rule can only justify a conclusion that matches its right side. Here, the right side is "shouldn't be subject to licensing requirement".
So this rule only has the power to prove a job shouldn't have a licensing requirement. Any answer that is trying to prove a job should is automatically wrong.
We could contrapose this and have
If it should be subject → incompetence is to licensing a threat
Thus, technically we could also use this rule to justify a conclusion saying that , but Apply the Principle questions have never worked that way in the past.
Whichever half of the rule sounds more "judgy", more normative or evaluative, should be on the right, because that's the conclusion they'll support.
In this rule, "should / shouldn't" is the most normative/evaluative language.
Goal
1. Check the conclusion first. If it isn't saying "shouldn't have licensing requirement", ditch it (at least on the first pass).
2. If the conclusion matches the right side of our rule, then we'll see whether the evidence in the answer choice establishes the left side of our rule, that someone being incompetent at doing job X wouldn't be posing a plausible threat to human health or safety.