Why is L not going third since Norton is going fifth?

Hi @raheimer@liberty.edu, Thanks for the question! You’re mixing up your necessary and sufficient clauses here. Remember, the third rule tells us that “L performs third ONLY IF N performs fifth.” X only if Y gets diagrammed X —> Y In other words, “only if” introduces the necessary, unlike “if,” which introduces the sufficient. So it should be diagrammed L third —> N fifth Which means that if N goes fifth, it doesn’t have to be true that L goes third. That’s a mistaken reversal. Hope this helps! Feel free to ask any other questions that you might have.

So, "only if" is necessary? Necessary gives me the idea that it is required in which case L and N work in the reverse. There is something I'm not grasping. Can you please explain this in a different way?