Why is “cannot” spelled as one word?

Why is “cannot” spelled as one word whereas other similar constructions such as “do not,” “will not,” “shall not,” “may not” and “must not” are spelled as two words (unless they are contracted as “don’t” and so on)?

(I know that languages are not always logical, so I would not be too surprised if there is no known reason for this. But I am asking this in the hope that there is some explanation.)

Solution 1:

Etymonline says:

c.1400, from can (v.) + not. O.E. expressed the notion by ne cunnan.

This doesn't really help, but it is a good starting point. The OED supplies the 1400 cite as follows:

Cursor M. (add. to Cott.) p. 959. 105 And þou þat he deed fore cannot sorus be.

It also defines cannot as:

the ordinary modern way of writing can not

Both Merriam-Webster and Wiktionary agree, by defining cannot as can not.

The Daily Writing Tips expand on this, their bottom line being:

There’s no difference in meaning between cannot and can not.

Funnily enough, they come to that conclusion after quoting two resources that say something slightly different:

The Washington State University language site:

These two spellings [cannot/can not] are largely interchangeable, but by far the most common is “cannot” and you should probably use it except when you want to be emphatic: “No, you can not wash the dog in the Maytag.”
AskOxford:

Both cannot and can not are acceptable spellings, but the first is much more usual. You would use can not when the ‘not’ forms part of another construction such as ‘not only.’

That last point is especially interesting. A very similar concern is being raised on the Wiktionary Talk page for cannot:

Maybe our current definition is wrong or incomplete. To me, "cannot" and "can not" are different words. "I can not sit down" means you have the option of not sitting down, but "I cannot sit down" means you don't have the option of sitting down. "can not" is rarely used baldly like that, it's usually used with "only" - "I can not only get to the school, I can get there in 10 minutes!".

To the extent that "can not" is an acceptable way, at all, of writing "cannot" (also written "can't") - I bet it is only because it is such a common mistake.

That post goes on to link to this blog:

My public school teachers said can not was the correct form, and that cannot was a corruption. A friend of mine from a previous generation was taught the opposite.
[...]
Here's the explanation: If I can not do something, then I can also do it. I can not write these words if I choose (and you may think I shouldn't), but I also can, and am, writing them. What I cannot do is know who will read them, or what they will think. I can imagine such things, but I'm limited by my experience and perceptions. So this is the rule: if you either could or could not do something, then you use two words, because you can leave out the second word if you so choose. If you could not do something no matter how much you desired or tried, then you use one word, cannot. There is no other option.

Sometimes both are true. Witness:

I cannot change the world.

I can not change the world.

Languagehat chimes in:

[The Merriam-Webster definition of cannot as can not] is appalling for two quite distinct reasons: from a copy-editing point of view because it implies that cannot and can not are interchangeable, and from a lexicographical point of view because it's a lousy definition. The definition of cannot should be either "the negative form of can" (as the AHD has it) or a periphrasis like "is not able to." The only context in which can not, two words, occurs is as an emphatic alternative: "You can do it, or you can not do it." In that case, it is clearly two separately spoken words, with the not given special emphasis, and equally clearly it means something very different from cannot, namely "have the option of not (doing something)." The only acceptable form for the unabbreviated negative of can (or, if you prefer, for the expansion of can't) is cannot, one word.

Take from all that what you will. Some of it is probably justification in hindsight, and certainly none of it actually explains how that 1400 cite came about and why this particular spelling prevailed over can not, canot, cant, and can non. As you say yourself, languages are not always logical.

Solution 2:

The double "n" is the best and likeliest explanation for why this convention long ago became established. Problems of clarity or pronunciation would seem to arise with the other modal forms you list (since they don't end in n) should you drop the space.

Solution 3:

One of the reasons is that "can not" is ambiguous while others are not, e.g. "will not", "do not", etc.

Solution 4:

This is that rare case when I disagree with Microsoft in its decision (Microsoft has decided to stick with "cannot" (dropping "can not") in its docs).

As I understood, the answer(s) to this question told about possibility of 2-word "can not" only.

I'd like to add on cases of impossibility of using one-word "cannot".

For example, this post tells that "cannot" can't be used in:

Cannot it be without repetitions? - incorrect
Can it not be without repetitions? - correct

And another post in the same thread tells:

I can't open the door.

meaning, because it is stuck
I cannot open the door.

meaning, because it is forbidden.
I can not open the door or I can open it.

meaning, I have a choice.

Solution 5:

_{Disclaimer

@RegDwight's answer, which I upvoted, is very well-written–as always–and unbiased; it allows visitors to make up their own minds by showing both sides of the argument.}

Is there a difference in meaning?

If one expects to find the difference in meaning between cannot and can not in a dictionary, they will be sorely disappointed. Dictionaries either state there is no difference in meaning or avoid mentioning it altogether

Dictionary.com

Usage note
Cannot is sometimes also spelled can not. The one-word spelling is by far the more common:

Interest rates simply cannot continue at their present level.

The contraction can't is most common in speech and informal writing.

Cambridge Dictionary says simply that cannot is the negative form of the verb "can" and under its American definition says: can not; to be unable or not allowed to. Oxford Learner's Dictionary, Macmillan Dictionary, Collins Dictionary do not add anything new to that essential definition. Wiktionary has a little more detail

Can not (am/is/are unable to)

Am/are/is forbidden or not permitted to

The American Heritage Dictionary, on the other hand, has this lengthy usage note, worth quoting in full.

The negative form of can.

Usage Note: The idiomatic phrase cannot but has sometimes been criticized as a double negative, perhaps because it has been confused with can but. The but of cannot but, however, means "except," as it does in phrases such as no one but, while the but of can but has the sense only, as it does in the sentence We had but a single bullet left. Both cannot but and can but are established as standard expressions.

The construction cannot help is used with a present participle to roughly the same effect as a verb form ending in -ing in a sentence such as We cannot help admiring his courage. This construction usually implies that a person is unable to affect an outcome normally under his or her control. Thus, saying We could not help laughing at such a remark would imply that one could not suppress one's laughter.

The construction cannot help but probably arose as a blend of cannot help and cannot but; it has the meaning of the first and the syntax of the second: We cannot help but admire his courage. The construction has sometimes been criticized as a redundancy, but it has been around for more than a century and appears in the writing of many distinguished authors.

The expression cannot (or can't) seem to has occasionally been criticized as illogical, and so it is. Brian can't seem to get angry does not mean "Brian is incapable of appearing to get angry," as its syntax would seem to dictate; rather, it means "Brian appears to be unable to get angry." But the idiom serves a useful purpose, since the syntax of English does not allow a logical equivalent like Brian seems to cannot get angry; and the cannot seem to construction is so widely used that it would be pedantic to object to it.

No mention whatsoever that cannot and can not have overtly or subtly different meanings.

I am of the opinion that if someone tells me

You can not / cannot park there

the meaning is absolutely identical, i.e. I am not allowed to park there.