Q: I heard you on NPR and I have a question: Why do people say “Let’s see if we can’t do this” instead of “Let’s see if we can do this”?
A: Let’s see if we can’t shed a little light.
In English, sentences that begin with “Let’s see if we can’t” or “I’ll see if I can’t” are routine, and most of us never stop to think about the negative construction. In fact, this kind of construction implies a positive outcome, not a negative one.
I haven’t found any sources that deal specifically with “see if we can’t,” but the linguist Otto Jespersen, in Essentials of English Grammar, writes about a similar phenomenon:
“A negative expression is often used idiomatically in such a way that the negative idea is weakened or even disappears totally.” He gives the example: “How often have I not watched him.”
Jespersen also notes that a question cast in the negative (“Isn’t that nice?”) actually implies a positive (“That is very nice”). I might add another example. “Won’t you join us?” really means “Will you join us?”
Elsewhere, Jespersen cites the expression “See if I don’t!” (Translation: “I will!”)
By extension, I think a sentence like “Let’s see if we can’t win” suggests that in fact we WILL win. On the other hand, the simple statement “Let’s see if we can win” is noncommittal. It doesn’t anticipate the outcome one way or the other.