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.

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. 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. If you could not do something no matter how much you desired or tried, then you use one word, cannot. 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.

I can imagine such things, but I'm limited by my experience and perceptions. What I cannot do is know who will read them, or what they will think. I can not write these words if I choose (and you may think I shouldn't), but I also can, and am, writing them. Here's the explanation: If I can not do something, then I can also do it. A friend of mine from a previous generation was taught the opposite. My public school teachers said can not was the correct form, and that cannot was a corruption. 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. '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!'. '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. To me, 'cannot' and 'can not' are different words. Maybe our current definition is wrong or incomplete. A very similar concern is being raised on the Wiktionary Talk page for cannot:

That last point is especially interesting. You would use can not when the вЂnotвЂ™ forms part of another construction such as вЂnot only.вЂ™ These two spellings are largely interchangeable, but by far the most common is вЂњcannotвЂќ and you should probably use it except when you want to be emphatic: вЂњNo, you can not wash the dog in the Maytag.вЂќД«oth cannot and can not are acceptable spellings, but the first is much more usual. The Washington State University language site: ThereвЂ™s no difference in meaning between cannot and can not.ДЇunnily enough, they come to that conclusion after quoting two resources that say something slightly different: The Daily Writing Tips expand on this, their bottom line being: The ordinary modern way of writing can notД«oth Merriam-Webster and Wiktionary agree, by defining cannot as can not. 105 And Гѕou Гѕat he deed fore cannot sorus be. The OED supplies the 1400 cite as follows:Д¬ursor M. This doesn't really help, but it is a good starting point.

Is cannot one word or 2