Passage Summary: The author claims that because every single thing people do is motivated by self-interest, self-interest must be the most important influence on our behavior.

Conclusion: Self-interest is the primary factor driving human behavior.

Reasoning: Every single human action is influenced by motives rooted in self-interest.

Analysis: This argument suffers from a subtle but significant shift in scope. Just because a specific factor is present in every instance of an action doesn't automatically make it the 'chief' or most important influence; other factors could be present less often but carry more weight when they appear. It is a bit like saying that because every car has tires, tires are the 'chief' reason a car moves, ignoring the engine entirely. Look for an answer that highlights this confusion between a universal presence and a dominant influence.