This was in last week's Parade magazine and I like Marilyn Vos Savant's answer.

What is yours?

why not

My answer is:
""

Not is a negative qualifier for an object otherwise assumed to be affirmative.
The opposite of "not" is not putting "not" there.

Obviously Marilyn Vos Savant's answer was more entertaining.

I would say that properly speaking, the opposite of "not in English" is null. Pick any sentence with "not" in it, and to form the opposite you simply remove the "not" and leave nothing in its place.

He is not here.
He is here.

I do not like broccoli.
I do like broccoli. (Or simply "I like broccoli," in which the null replaces two words.)

This is rather common. No in Spanish, nye in Russian, ne... pas in French, bu in Mandarin. Remove the negative from the sentence and leave the place blank, and you've got the positive.

Still this rule is common but not universal. In Japanese:

Ni Hon go ga deki masu. I speak Japanese.
Ni Hon go ga deki masen. I do not speak Japanese.

"So" is a word expressing emphasis or denial, not simple negation:

You are not smart.
I am so!

Other words fulfill the same purpose, such as "too" informally and "indeed" formally. But none of them is truly the opposite of "not."

If I said offhandedly, "I have not been to Poland," you would surely respond "I have been to Poland," with no need for a word of emphasis or denial.

Marilyn has the world's highest verified IQ score, but that does not make her a linguist any more than it would make her a chemist.

What was her answer?

So.

all relative.

But it was from a readers' poll. She does that every week. She writes a weekly column in Parade magazine, which is probably the largest-circulation magazine in the anglophone world because it is distributed with about half of the Sunday newspapers in America.

The people who answer her polls are not a representative demographic. For example, a couple of weeks ago the question was: "What are the ten worst inventions in human history?" I don't remember the whole list, but it included high-heeled shoes, cell phones, neckties, leaf blowers, jet skis, and car radios with subwoofers. Obviously not the results of a poll of young people.

(to undo one, you apply the other)
'start' vs 'stop'
'rotate left' vs 'rotate right'

to undo a 'not' you reapply 'not'. Therefore it's its own opposite.

Huh? So the opposite of "Rabbits are not rodents" is "Rabbits are not not rodents?"

Actually, it is.
It's sloppy and cumbersome, but it is accurate.

The opposite of "not" can be:

"is".

Jonny is not here.

Jonny is here.

"Yes"

"Not I, Horace!"

"Yes I, Horace!"

"am"

"I am not John Wilkes Boothe"

"I am John Wilkes Boothe".

Et cetera, et cetera.

"is" i agree.

Truly

The opposite of not is not. positive times a negative is negative. Positive time a positive is positive. The negative of a negative is negative. Simple math really.

It is certainly not 'is'.

You cannot replace 'not' with 'is', or add 'is' elsewhere in the above sentence and leave it valid.

'Not' is a curious word to try and find an opposite for because, in fact, its very meaning concerns opposites. It is its own opposite!

My explanation:

For a word (or thing in general) to have an opposite it must have an element of polarity to it. Words with polarity such 'left', 'negative', 'hot', 'north', 'happy' all have opposites for that reason.

'Not' does, in a sense, have polarity - however it does not specify which polarity - it means instead 'swap the polarity from whatever it is'. Therefore it can have no opposite other than itself.

edit - maybe not completely accurate as 'not' can also mean excluding, e.g. 'not north' does not make 'south', but I'm sure the point is clear

However, the opposite of 'is not' is 'is'!

So what's wrong with "truly" ?

that's truly not correct. Or it is not truly correct.