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Could you tell me how the following sentences are different?

  1. Alice and Bob are not both in the room.
  2. Alice and Bob are both not in the room.

Updated:

Could you divide the sentences into a set of grammatical chunks and point me how each chunk affects to another?

Doofah
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  • Hi Doofah, welcome to EL&U! Please wait a day or two before selecting an answer. You may get several more answers with other ideas or different advice. But people may not bother to write you another answer if you've already selected one! :-) – Araucaria - Him Jun 29 '16 at 10:27
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    Hi, Araucaria! Thank you for your advice. Okay, I'll wait for other answers to come. :) – Doofah Jun 29 '16 at 11:46
  • Hi Doofah, if you're learning English, you may find our sister site English Language Learners helpful. – TrevorD Jun 29 '16 at 12:27
  • Hi TrevorD, yes, I'm learning English but the original question was coming from a pure logical/mathematical textbook. I was simply confused by the both sentences so I decided to ask here. ;) – Doofah Jun 29 '16 at 13:01

2 Answers2

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They are logically different as they are asserting different things.

  1. Either Alice or Bob, or neither, are in the room, but not both.

  2. Neither Alice nor Bob is in the room.

Edit for your additional question (what is the logical negation of each):

  1. "Both Alice and Bob are in the room."

  2. "Either Alice or Bob, or both, are in the room."

Jeffrey Kemp
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  • You could possibly argue that the logical negation of "Alice and Bob are not both in the room." is "Alice and Bob are both in the room." I can argue this only because "logical negation" is actually quite vague in this context. – Max Williams Jun 29 '16 at 08:15
  • Hi, Jeffrey, I think it is better to use "is" instead of "are" in No. 1 and 2 sentences. What is your thought? –  Jun 29 '16 at 08:22
  • Thank you, Jeffrey! (Unfortunately I've deleted the additional question by accident though...) – Doofah Jun 29 '16 at 08:22
  • Only #2 needs the singular form. Corrected. – Jeffrey Kemp Jun 29 '16 at 08:26
  • Okay, I get both "Both Alice and Bob are not in the room." and "Alice and Bob are not both in the room." are logically identical. So would the later one sound natural for native speakers? Would there be any intentional emphasis on that usage? – Doofah Jun 29 '16 at 09:23
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    @Doofah (3) "Both Alice and Bob are not in the room." and (1) "Alice and Bob are not both in the room." are *not* logically identical. (3) *is* logically identical to (2) "Alice and Bob are both not in the room." Neither (2) nor (3) sounds natural (to this native British speaker): as the above answer already suggests, I would say "Neither Alice nor Bob is in the room.". – TrevorD Jun 29 '16 at 12:37
  • Neither sound very natural because it's a very unusual kind of assertion to make. This is English, not a programming language :) Normally one would just say either "Alice is in the room, but Bob isn't" or "Alice isn't in the room, but Bob is.", or, more simply, "I don't know whether Alice or Bob is in the room or not." – Jeffrey Kemp Jun 29 '16 at 12:37
  • Agreed with TrevorD, "Both Alice and Bob are not in the room" is not logically equivalent; and it is a very awkward construction. It sounds almost like a double negative (although it isn't). – Jeffrey Kemp Jun 29 '16 at 12:39
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Both Alice and Bob are not in the room means they are together not in the room while Alice and Bob are not both in the room means maybe one of them are in the room.

Sky Sunshiner
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