L2 HW Complex Diagramming Drill

[MandeeC]

In the reading, either/or is described with "Either John or Jack will attend..." being diagrammed as ~John-->Jack contrapositive ~Jack--> John. In the drill, the sentence "Either Jones or Kim will win the election." is diagrammed as ~Jones-->Kim contrapositive ~Kim-->Jones.
This means that if Jones wins, nothing happens. I understand that it's possible for both to win, but would it be wrong to diagram it as Jones-->~Kim? If so, why would it be wrong and what is the reason for negating the first term in those scenarios?

[MandeeC]

I believe I may have answered my own question in my post as to whether J-->~K would be wrong, but is there a way you can explain the logic of negating the first term? Something that explains how and why that is the scenario in which it translates correctly?

[Claire Horan]

Hi MandeeC,

In our everyday lives, when we say "or," we usually mean "either x or y, but not both." In logic, "or" can mean "either x or y, but not both" or it can mean "x or y or x and y." In other words, "or" includes the possibility of "and." A statement in logic that "it will rain or snow today" includes the possibility that it may rain AND snow. This possibility is the reason that we don't diagram the other way that you suggest.

Usually, on the LSAT, when either/or language is used, the question will clarify by saying "or both" or "but not both," so this is generally not a problem. But, if a logic game has a rule that "x or y must be present," it's okay for both x and y to be present. So, it's important to pay close attention to the exact language used.

Notice, though, that in your example of the election, "but not both" is implicit if we also know an additional fact, that only one person can win an election.

Let us know if you still have questions!