Passage Summary: Filling a tooth always does some damage, and since many cavities never actually reach the painful stage, we shouldn't fix them unless we absolutely have to.

Conclusion: Dentists should refrain from filling cavities unless the decay is about to reach the tooth's nerves.

Reasoning: Filling a cavity causes guaranteed damage to healthy tissue, whereas many cavities never actually cause harm if left alone.

Analysis: The argument moves from a set of facts about risks and benefits to a professional 'should' statement. To justify this, we need a principle that connects these dots—likely something that says you shouldn't perform a procedure that causes certain harm unless it is necessary to prevent a greater harm. Look for a rule that prioritizes avoiding the guaranteed harm of the procedure over the potential harm of the condition.