I came across this episode of Uncommon Knowledge some time ago which features the economist Thomas Sowell. He makes the claim that David Hume's first language was not English. He then sort of fumbles a bit when pressed on it saying, 'it was whatever was spoken [in Scotland] in those days.' (video is bookmarked at the beginning of this interaction)
That statement rang a bit strange to me so I decided to investigate. The Stanford Encyclopedia of Philosophy as well as a number of other sources I looked at (including Hume's own writing) made absolutely no mention of English being his second language, nor any mention of Hume speaking Scots Gaelic-- though of course plenty on his fluency in Latin and Greek. That said though, none of them explicitly mention English being Hume's first language either.
It seems to me that something of this nature would of course be mentioned in any serious source if it were true, and so I put the question to you: Was David Hume's first language English?
Ironically, this episode of Uncommon Knowledge centers around intellectuals bloviating on topics outside their precise area of expertise and being quite incompetent when doing so. Perhaps Sowell was just making a subtle joke on his point, or perhaps he was proving it himself.
English was not David Hume's first language, though Hume himself would have disagreed.
David Hume was born in the Edinburgh of 1711, a city whose predominant language was still Scots.
Scots is the sort of language that straddles the borderline between language and dialect, and you could call it either a particularly variant dialect of English or a language closely related to English. Linguists tend to side with the latter, though. You might best recognize it as the language of Auld Lang Syne (spelling revised to fit Scots pronunciation better, since Burns used an Anglicized orthography):
Shid ald akwentans bee firgot,
an nivir brocht ti mynd?
Shid ald akwentans bee firgot,
an ald lang syn?
Since before Scotland was even Scotland, the Edinburgh area had always spoken Scots or its Old English ancestor. By the thirteenth century, Scots had become the language of most of the Scottish Lowlands and the dominant language of the royal court. It evolved largely separately from its southern sister language for the next few centuries, and by the 1490s a Spanish ambassador was saying that Scots was as different from English as "Aragonese from Castillian."
In the seventeenth century, though, the Scottish dynasty of the Stuarts gained the English throne, and Scotland came to be ruled from a king in London. Written Scots was then progressively Anglicized. And by the Scottish Enlightenment of the eighteenth century, Scots was largely considered a quaintly provincial variety of English. Perhaps the most telltale feature of this new conception of Scots as a sort of English is the apologetic apostrophe. For example, the Scots word meaning "full", pronounced like "fou" and originally spelled as such, came to instead be written as fu' with an apostrophe in order to make the connection to the "correct" English word more obvious. Many Scottish Enlightenment thinkers considered Scots an impediment to the progress of Scotland and sought to implement "correct" English in their speech and writing.
Hume himself was a native speaker of Scots—and terribly ashamed to be one. In 1757, he marvels at the ingenuity of his countrymen, who have succeeded in being "most distinguish'd for Literature" despite the fact that:
in our Accent & Pronunciation, [we] speak a very corrupt Dialect of the Tongue which we make use of.
In a letter he wrote in 1754, Hume laments:
Notwithstanding all the Pains, which I have taken in the Study of the English Language, I am still jealous of my Pen. As to my Tongue, you have seen that I regard it as totally desperate and Irreclaimable.
George Norvell, a relative of Hume, remembers:
Notwithstanding his opportunitys, his conversations in Britain, his mastery of several languages... in Common conversation, he retained the accent, expression, and vulgarity of his paternal stile on the Banks of the White Water & Tweed, in such a degree that you would have imagined he had never conversed with any person but the commonest farmer in the Merse or ever sett foot out of the parish of Chirnside...
If I'd ask what he was saying [when Hume was concentrating on a problem, Hume would respond in Scots, saying] "Lord canna ye let a Body amuse themselves without always clattering, I did nae ken ye was there and nae matter ye had nae bus'ness to meddle wi me."
Hume was in fact famous for publishing a list of "Scotticisms" to avoid in 1752, the first of a flurry of such anti-Scots writings in the late eighteenth century. And when Hume passed away, Lord Monboddo quipped that he had died confessing, not his sins, but his Scotticisms.
This answer draws largely on Ch. 27 of Mossner's The Life of David Hume.