----------By citing Johnson's, his aim is to demonstrate how the concept of "English" has changed over time. Horobin continues:
ENGLISH. adj.
Belonging to England; thence English is the language of England.(Dictionary of the English Language)
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----------Johnson's definition "hardly captures the diversity and complexity," the one that is not easy to apply to in our present times. It tells how the meaning of the word changes quite in an unpredictable way. Even the gigantic figure in the field of lexicography would not have doubted English is the language of England!
"Samuel Johnson's straightforward identification of English as the language of England hardly begins to capture the diversity and complexity of the language's use in the twenty-first century."
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Perhaps the closer to our sense of "English" is the one that first appears in 1910 according to the OED:
----------Doctor Johnson would be surprised to know that English is, above all, "a count noun"!
2d. As a count noun: a variety of English used in a particular context or (now esp.) a certain region of the world; (in pl.) regional varieties of English considered together, often in contradistinction to the concept of English as a language with a single standard or correct form.
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One would say that the time is pretty much closer to the end when people takes English "as a language with a single standard or correct form," as the OED puts it.
These are the ideas that lead up to the very interesting question posed in "Old English, a different language?
Works cited:
Horobin, Simon. How English Became English: A Short History of a Global Language. Oxford University Press, 2016.