Only in the passage

@ Ben, Great question. It is the other way around - "only" or "only if" typically introduces a necessary condition, and "the only" introduces a sufficient condition. "If and only if" introduces both. Only an expert could understand: IF understand THEN an expert IF NOT an expert THEN not understand Compare it to: The only president to serve two nonconsecutive terms is Grover Cleveland. IF nonconsecutive terms THEN Grover Cleveland IF NOT Grover Cleveland THEN NOT nonconsecutive terms. And compare it to: A number is divisible by 9 if and only if the sum of its digits is divisible by 9. IF sum divisible THEN number divisible IF number divisible THEN sum divisible Does this make sense? Let me know if you have any further questions.