I usually begin my posts with the phrase 'A correspondent writes...'. This time it has to be 'A Correspondent writes...' For the query comes via Her Majesty the Queen.
No, it is not the case (I imagine) that Her Majesty reads this blog. What happened was this. A Japanese professor, anxious to check on a question of English grammar in the King James version of the Bible, thought that the best way of deciding the matter was to write directly to the Queen. (I think that's wonderful, that someone should think to do such a thing.) Her Majesty's correspondence officer, observing that 'this is not a matter on which The Queen would comment', forwarded the letter to the English-Speaking Union (the Duke of Edinburgh is its patron), and they in turn passed it on to me. So I guess this is the next best thing to having a royal appointment! Maybe there will be a post of Grammar Laureate one day.
It's an interesting issue. The correspondent had noted an unusual modal auxiliary use in Exodus 10.5, where it is said that locusts will cover the face of the earth so that 'one cannot be able to see the earth'. He suggests - very plausibly, to my mind - that this has to mean, 'it is not possible that one is able to see'. And he comments that the expression is 'unnatural'.
It certainly wasn't unnatural in the 16th century. There are several examples of can/cannot co-occurring with be able to. A couple occur in Shakespeare. In The Winter's Tale (5.2.25), the Second Gentleman observes: 'Such a deal of wonder is broken out within this hour that ballad-makers cannot be able to express it.' And in Antony and Cleopatra (1.4.78), Lepidus says:
I shall be furnished to inform you rightly
Both what by sea and land I can be able
To front this present time.
It seems that can was being treated just like may, shall. and others, all of which can precede be able to - I may be able to go, They should be able to go, and so on.
As the modal system developed, the association of can with be able to fell away, for two reasons. There was an overlap of meaning between the ability sense of can and that of be able to, which made can be able to seem tautologous. And be able to came to fill the gap of expressing ability whenever a nonfinite construction required it: one says to be able to talk, not to can talk. The two expressions thus complemented each other, and this made it less likely that they would co-occur.
But the usage didn't die away completely, especially in negative expressions. I have heard can't be able to in some regional dialects. Nor did it entirely disappear from Standard English. If you do a Google search for cannot be able to there are 3000 or so examples. Now, admittedly many of these are likely to come from people for whom English is a second or foreign language, and where learning levels are unclear, but there are several cases where the text has been written by native speakers in Standard English. Here is an example, from Articles Base:
'E-mail notifications should be provided to inform users when they are required to perform a task or need to be aware of an event. This should be able to be deactivated but at the same time there must always be some alternative form of notification that cannot be able to be deactivated.'
It would be interesting to hear if the usage is known by readers of this blog.