When does "either" mean "and"? (LGB pg 153)

[peregrinus]

Hi,

This is in regard to the game on pg 153 of the logic games bible (Game #2 from October 2004). Specifically this is with respect to rule #5, which uses terminology that effectively says something like C is newer than either A or B. This is taken to mean that C is newer than A, as well as newer than B i.e. (C < A) & (C < B). My understanding was (C < A) | (C < B).

This terminology seems highly ambiguous. I've seen similar terminology in another book where a question is phrased this way: R could invite either A or B if he invites which of the following pair of people? Since one of the rules of the game was that A and B could not be invited together, I took this to mean it should be possible to invite one of the two. However turns out it meant that it should be possible to invite A as well as possible to invite B, just not both.

Any clarifications on how to interpret the meanings of "either ... or" in such situations?

Much appreciated.

[Nikki Siclunov]

Great question! Indeed, "either/or" can be a confusing conjunction and often creates some ambiguity. There are some solid rules (describes below); that said, you need to look at the context in which the phrase is used.

Typically, a rule such as "either A or B must be selected" is inclusive: it allows for the possibility of choosing both A and B (unless the rule specifically states "either A or B must be selected, but not both").

However, compare this rule to the following:

Mary has a higher LSAT score than does either Kate or Jane.

Whenever this wording is used ("than does either"), we can infer that Mary's LSAT score is higher than either of the other two girls': i.e. Mary has a higher score than both of them. "Either" in this case requires the use of "or" so that the sentence is grammatically correct, but the rule as a whole implies both!

Let's take a few examples.

In the constructions below, "either...or" means both:

• A arrives earlier than does either B or C.
  Jack is a better chess player than is either his mentor, or his opponent.

By contrast, the following constructions, which mean either/or (but not necessarily both):

• A arrives before B or before C.
  Jack is either a better chess player than his mentor, or a better chess player than his opponent.

Finally, the following constructions mean either/or (but not both):

• A arrives before B or C, but not before both.
  Jack is either a better chess player than his mentor, or a better chess player than his opponent, but is not better than both of them.

Does this help? Let me know.

[Dave Killoran]

Hi Peregrinus,

To add to Nikki's comments, this is a question that has arisen before (and it's a good point you ask about), so if you get a chance, check out the prior discussion, which is at: http://forum.powerscore.com/lsat/viewto ... 864&p=1789.

Please let me know if that helps. Thanks!