I know there are quite a few threads about in case vs. lest, but I think none has seriously questioned the presumed interchangeability of the following:
- Take an umbrella with you in case it rains.
- Take an umbrella with you lest it (should) rain.
1 is definitely idiomatic, but 2, according to the dictionary definition of lest, seems problematic, even though some native speakers regard it correct, however uncommon lest is.
lest means "so as to prevent something from happening." (The other use basically means "that" and is used following adjectives denoting fear, worry, etc., and is outside the scope of the present discussion.)
But having an umbrella with you will not prevent the rain from happening. If so, would you still consider the use of lest in 2 correct?