Some linguists speak of the uselessness of not splitting infinitives. They offer as a counter example the famous "to Boldly Go". And they claim that the rule is a mindless mimicry of infinitives in earlier languages, which are expressed with one word.
I am not so sure that the mimicry is either mindless or useless. And I do not pretend to know why the rule developed.
But at any rate, it seems to me that the rule is rational: I think that in most cases it's wise not to separate an infinitive's components. An adverb or adverbial phrase is notionally dependent on its object. Without the object, an adverb has no concrete meaning in so far as the context of the housing grammatical element (the phrase or clause).
So with a split infinitive and the reader arriving at the adverbial element before its object, the mind does not have anything notionally concrete to hold onto.
When the reader, then, moves on to the object, the mind uses the non-concrete data contained in it's memory
to define the notionally concrete object, thereby arriving at a more complex concrete idea.
In other words: (non-concrete data = '0'; concrete data
= '1'; complex concrete idea = '2')
With a split infinitive, at step one (reading the adverb) the mind stores '0', and at step two (reading the object) he arrives, through adding '0' to '1', at '2'.
With a unified infinitive, at step one (reading the object) the mind stores '1', and at step two (reading the adverbial element) arrives, through adding '1' to '0', at '2'.
Therefore, if one holds, as I do, that it is more taxing on the mind to hold onto non-concrete data ('0')
than it is for it to hold onto concrete data ('1')--it
follows that unifying the elements of the infinitive is a more efficient way to communicate content, since it either does not require the readers mind to hold onto non-concrete data ('0') or requires it for less time.
To illustrate: The reader's mind being what it is,
a writer can get away with "To boldly go". But it would seem more problematic for him to write, "To--boldly and swiftly, eyes frontward and never looking back--go.
"To go boldly and swiftly, eyes frontward and never looking back" is more easily understood.
Accordingly, it would seem logical to place adverbial elements after their respective objects generally; I do not know the rule on that.
Also, it would seem (?) to follow that adjectival elements should follow their respective objects as well. We rarely if ever do that it English, but, interestingly, that is the norm in Hebrew.
The following was added after the original posting:
After looking at the post, it occurred to me that according to the forgoing explanation not splitting infinitives is merely an instance of not placing adverbial elements before their respective objects.
In reality, however, splitting an infinitive is worse than placing (at least) one adverb before its object, as the "to" of the infinitive is at least as non-concrete as the adverb itself. And since, when reading a split infinitive, it takes as least one more step to resolve the non-concrete "to", the mind has to hold onto it for that much longer.
Therefore, splitting an infinitive is, according to this explanation, roughly equivalent to placing two adverbs before their object.
However, if "to" is even less concrete and thus harder to hold onto than an adverb, as I think it is, then splitting an infinitive is worse.
After reading the sample phrases mentioned earlier in the post, it also occurred to me that a split infinitive presents a unique difficulty in combining the language components to render their complex referent: (the 'to' element of the infinitive = '0'; the verb element of the infinitive = '1' ; the adverbial element
= '2'; the complex idea = '3')
Resolving "To go boldly" requires the mind to add thusly: '0' + '1' + '2' = '3'. At step two the mind is able to perform '0' + '1', and thereby needs only to hold onto the product. At step three, the mind adds thusly: (presolved product of '0' + '1') + 2 to arrive at '3', the complex idea.
Similarly, resolving "boldly going" requires the mind to add thusly: '1' + '2' = '3'
Resolving "to boldly go", however, requires the mind to add '0' + '1' + '2' = '3', but receives the data in the order of '0' + '2' + '1' = '3'. So not only must the mind hold onto '0' + '2' as separate data entities until step three, at step three it must rearrange the data into '0' + '1' + '2' to arrive at '3'.