Date:  Tue, 13 Jan 1998 11:44:32 -0500
From:  Larry Horn <laurence.horn[AT SYMBOL GOES HERE]YALE.EDU>
Subject: Re: A question of the use of "or".

>I have a question about the use of "or" in the following sentence.
>
>I left the hotel; or I would have missed the train.
>
>Of course, in this sentence "otherwise" is correct. But what do you
>think of "or" in this sentence? Here "or" means "and if I had not". I
>wonder if I should change the semi-colon before "or" into a comma in
>this sentence.
>
>In Quirk et al.'s _A Comprehensive Grammar of the English Language_
>(1985), I found the following example:
>
>They (must have) liked this apartment, or they wouldn't have stayed so
>long.
>
>I would like to have your opinion about this matter. Thanks in
>advance.
>
>Hideho Ida hida[AT SYMBOL GOES HERE]mail.doshisha.ac.jp

You're right in taking this to be an "or" with essentially the meaning of
"and if not". More fully, "p or q" here is equivalent to something like
  "p (because) if not-p, q"
where q is either something to be avoided or something incompatible with
the assumed context. The use of "or" here plays off the logical
equivalence
  [p v q] <--> [~p --> q]
Interestingly, though, I find "either...or" impossible in this context,
while "else" is pretty good:
  (*Either) I left the hotel, or (else) I would have missed the train.
As for punctuation, my intuition says comma, not semicolon.

Larry