Sophie:

The following is a direct quotation from the Cambridge International
Dictionary of English
<http://dictionary.cambridge.org/define.asp?key=lest*1+0>:

lest / conjunction / LITERARY
  in order to prevent any possibility that (something will happen)
  They were afraid to complain about the noise lest they annoyed the people
next door.
  Lest you think the film is too violent, I must assure you that it is not.

That online dictionary didn't show an entry for "in case," but here's one
from Merriam-Webster Collegiate Dictionary
<http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=in+case>:

   in case
            Function: conjunction
            Date: 14th century
            1 : IF <in case we are surprised, keep by me -- Washington Irving>
            2 : as a precaution against the event that <carries a gun in
case he is attacked>

In other words:

"X lest Y"  means  "X so that not Y"
"X in case Y"  means  "X because maybe Y"

In still other words:
A "lest" sentence tells what you do so that something bad doesn't happen.
An "in case" sentence tells what you do as a precaution in the event that
something bad actually does happen.

This being my third posting in 24 hours, I would like to leave any further
discussion on this topic to others.

Dick Veit

At 09:04 PM 07/14/2001, Sophie wrote:
>Dick, how do you come at a distinction in meaning between `lest' and `in
>case'? I cannot find any lexicographic ground to support it. Both of these
>logical operators hypothesise an event: neither presumes its inevitability
>or the circumstance of its prevention. That is precisely why they head
>subjunctive-mood sequences. Your paraphrasing:
>
>D, (lest):  Because I want to make sure I don't slip and fall, I will hold
>on to the hand rail
>
>has turned 'lest' into a causal logical operator and thereby diverted the
>sense the subjunctive is in place to achieve.
>
>The semantic template to represent the subjunctive mood of the sentence
>under scrutiny is this:
>
>`I hold on to the rail and [I hypothesise the possibility that] I slip and
>fall'.
>
>It cannot possibly be `I hold on to the rail therefore I will not (or: in
>order that I do not) slip and fall. If this were a possibility then the
>lest/just in case headers, natural headers of subjunctive-mood sequences,
>would simply not be in use.
>Sophie
>
>----- Original Message -----
>From: Richard Veit, UNCW English Department <[log in to unmask]>
>To: <[log in to unmask]>
>Sent: Sunday, July 15, 2001 12:34 AM
>Subject: Re: "in case" and "lest S should"
>
> > At 12:32 AM 07/14/2001, Sophie Johnson wrote:
> > >... Indeed, `lest' and `just in case' are synonymous expressions. So you
> > >are right: both C and D correctly fill the gap in the exam sentence. And
> > >you are also right in that `lest' is archaic. On that basis, C would have
> > >been the better answer.
> >
> > I must disagree with Sophie. "Lest" and "just in case" are not synonymous.
> > "Lest" means "to prevent X from taking place." "Just in case" means "in
>the
> > event that X should actually take place." Very different. I agree that
>both
> > C and D could be said, but D seems the more likely answer. Here are
> > equivalent statements (not exact paraphrases by any means):
> >
> > C. (just in case):  When I slip and fall, I want to be holding on to the
> > hand rail.
> > D, (lest):  Because I want to make sure I don't slip and fall, I will hold
> > on to the hand rail.
> >
> > Dick Veit