@ClaudiaMurvartian-Rhim In LSAT language, I guess it is just simply not true that "either... or" negates the existence of both. You can review the first lesson on either (with the class schedule example) to apply that theory here. I find that example easy to understand because it's easy to visualize.

I think what makes this example easy to digest as well is that you can think of AF and JF as completely separate events. Who's to say they didn't both just have really bad days and fail at their mission? Either way, we know one of those things has to fail for the mission to not succeed. That doesn't mean that they can't both fail.

@OliverJakacki We are creating the assumption that the bomb is the assassination attempt, but since our evaluation doesn't call into question the validity of our assumptions (only the validity of our argument), we can effectively move on.

@karendn yes. since we have no confirmation that a sufficient condition was met for the speech to be delivered, we don't know whether it ever happened. since we know the speech is a necessary condition for the vote, and we have no understanding about the outcome of the speech, we have no confirmation that the necessary condition was met.

@sydneys1942 It says in the written portion of this lesson that if the LSAT wants to communicate the exclusive meaning, it will be explicit. So if you simply had the sentence

"Jon must enroll in Econ 101 or Polisci 101 this term"

there is no condition that makes it impossible for him to do both, only a condition that makes it impossible for him to do neither. For example, we don't know how many courses he can select, we don't know when the courses are. We don't know anything other than that he will for sure take one of the classes. This sentence would differ from

"Jon must only enroll in one class between Econ 101 and Polisci 101 for this term, but not both."

or something along those lines. This still means he will end up in either Econ 101 or Polisci 101, because it is required that he takes one or the other. But the preceding statement says he can only pick one. In this case, it is explicit that we are to interpret the exclusive or meaning.

Tiger: High heels without any support are used for style and fashion. Not all shoes are meant for comfortable wear.

Mickey: My local baker must order his groceries. In order to run a bakery, you must shop in specialty stores with permit only or online-order groceries to have ingredients. My local baker doesn't have a permit, but he always has groceries, so he must order his groceries online.

Mr. Fat Cat: My favorite keychain is missing from my purse, though my other keychains are there. My little sister is holding my purse in the air, playing with the straps. My hypothesis is that my little sister took the keychain off my purse, having taken it because she favors it.

the sun was too bright and my eyes hurt all day.

premise: the sun was too bright.

conclusion: my eyes hurt all day.

the premise, that the sun was too bright outside, is why they want me to believe (support) that my eyes hurt all day, which is what happened!