As part of the discussion about Donald Trump’s lack of fitness for the presidency, some have argued that his linguistic capacities have worsened, suggesting significant cognitive decline.

In interviews in the 1980s and 1990s, Mr. Trump speaks in complete sentences, using mature vocabulary and expressions. There aren’t the endless digressions that make his current mode of expression a kind of vocal fantasia. America has certainly never experienced a commander in chief who expressed himself in this fashion. In public, at least.

Donald Trump in television appearances in 1987 and 2015. (CBS; C-Span) Hover for sound "The Late Show with David Letterman," 1987 (CBS) Hover for sound Sun City, South Carolina, July 21, 2015 (C-Span)

However, the distinction between public and private speech is key here, so I am unconvinced that his current speech patterns can be analyzed as evidence of dementia. Instead, they’re characteristics of casual speech as it has always existed.

It is easy to forget how much casual speech in general differs from writing. We tend to imagine our speech is tidier than it often is. The complete sentences and logical throughlines of writing are a stylization of speech, rather than a mirror image.

Take this famous Trump utterance from July 2015.

Hover for sound Look, having nuclear – my uncle was a great professor and scientist and engineer, Dr. John Trump at M.I.T.; good genes, very good genes, O.K., very smart, the Wharton School of finance, very good, very smart – you know, if you're a conservative Republican, if I were a liberal, if, like, O.K., if I ran as a liberal Democrat, they would say I'm one of the smartest people anywhere in the world – it’s true! – but when you're a conservative Republican they try – oh, they do a number – that’s why I always start off: "Went to Wharton, was a good student, went there, went there, did this, built a fortune” – you know I have to give my life credentials all the time, because we're a little disadvantaged – but you look at the nuclear deal, the thing that really bothers me …

Real speech comes, on average, in packets of 10 or so words at a time, rather sloppily juxtaposed. Rapid, spontaneous talk makes more use of parataxis – the stringing of simple clauses together, such as in this segment in bold type:

Look, having nuclear – my uncle was a great professor and scientist and engineer, Dr. John Trump at M.I.T.; good genes, very good genes, O.K., very smart, the Wharton School of finance, very good, very smart – you know, if you're a conservative Republican, if I were a liberal, if, like, okay, if I ran as a liberal Democrat, they would say I'm one of the smartest people anywhere in the world – it’s true! – but when you're a conservative Republican they try – oh, they do a number – that’s why I always start off: "Went to Wharton, was a good student, went there, went there, did this, built a fortune”– you know I have to give my life credentials all the time, because we're a little disadvantaged – but you look at the nuclear deal, the thing that really bothers me …

In writing, this would likely be rendered using hypotaxis, which entails clearer subordinate clauses. The same sentence would be written as: “My uncle Dr. John Trump, who was a professor at M.I.T., had very good genes, which lent him considerable intelligence.”

Real speech tends to be fundamentally subjective rather than objective, and is therefore decorated with what linguists call pragmatic words and constructions, expressing attitude rather than content:

Look, having nuclear – my uncle was a great professor and scientist and engineer, Dr. John Trump at M.I.T.; good genes, very good genes, O.K., very smart, the Wharton School of finance, very good, very smart – you know, if you're a conservative Republican, if I were a liberal, if, like, O.K., if I ran as a liberal Democrat, they would say I'm one of the smartest people anywhere in the world – it’s true! – but when you're a conservative Republican they try – oh, they do a number – that’s why I always start off: "Went to Wharton, was a good student, went there, went there, did this, built a fortune” – you know I have to give my life credentials all the time, because we're a little disadvantaged – but you look at the nuclear deal, the thing that really bothers me …

Mr. Trump also turns the lens in on himself when he switches from “you” to “I” within the passage, another indication of the fundamentally subjective nature of casual speech, even if his ample use of it suggests an especially strong focus upon the self.

Look, having nuclear – my uncle was a great professor and scientist and engineer, Dr. John Trump at M.I.T.; good genes, very good genes, O.K., very smart, the Wharton School of finance, very good, very smart – you know, if you're a conservative Republican, if I were a liberal, if, like, O.K., if I ran as a liberal Democrat, they would say I'm one of the smartest people anywhere in the world – it’s true! – but when you're a conservative Republican they try – oh, they do a number – that’s why I always start off: "Went to Wharton, was a good student, went there, went there, did this, built a fortune” – you know I have to give my life credentials all the time, because we're a little disadvantaged – but you look at the nuclear deal, the thing that really bothers me …

Casual speech tends towards the repetitious, as we seek to indicate sincerity and make sure our meaning has gotten across. Mr. Trump, because he speaks casually, does a lot of repeating.

Look, having nuclear – my uncle was a great professor and scientist and engineer, Dr. John Trump at M.I.T.; good genes, very good genes, O.K., very smart, the Wharton School of finance, very good, very smart – you know, if you're a conservative Republican, if I were a liberal, if, like, O.K., if I ran as a liberal Democrat, they would say I'm one of the smartest people anywhere in the world – it’s true! – but when you're a conservative Republican they try – oh, they do a number – that’s why I always start off: "Went to Wharton, was a good student, went there, went there, did this, built a fortune” – you know I have to give my life credentials all the time, because we're a little disadvantaged – but you look at the nuclear deal, the thing that really bothers me …

This passage also has a kind of coherence. Mr. Trump starts out addressing the nuclear deal with Iran, but then interjects some insights about his elevated qualifications for discussing the subject and his resentment of a purportedly biased media that forces him to mention them – and then he does get back to the nuclear subject. Elegant? No, but not demented, either. Facial expression and context, which print obscures, made Mr. Trump’s meaning clear.

Nuclear Uncle Intelligence Republicans Democrats Republicans Intelligence Republicans Nuclear Nuclear Uncle Intelligence Republicans Democrats Republicans Intelligence Republicans Nuclear

Two and three decades ago, Mr. Trump spoke to David Letterman and Rona Barrett in the quietly composed phrasing we expect of public figures expressing serious thoughts. So why does the same man now toss off word salad?

Because he can.

The younger Mr. Trump, albeit as self-obsessed as now, was not yet a rock star, and he had a businessman’s normal inclination to present himself in as polished a manner as possible in public settings. Especially as someone who grew up in the 1950s, when old-school standards of oratory were still part of the warp and woof of American linguistic culture, Mr. Trump instinctually talked "up" when the cameras were rolling. To him, cloaking his speech in its Sunday best would have been part of, as it were, being a gentleman.

However, for him this would always have been more stunt than essence. Since he is someone who neither reads nor reflects, his linguistic comfort zone has always been the unadorned.

At a certain point, Mr. Trump became the man who felt – and was comfortable saying publicly – that he could shoot someone on Fifth Avenue and retain his supporters' allegiance. Someone with that mind-set, especially a sybaritic person unaccustomed to sustained effort, has no impetus to speak in a way unnatural to him in public.

Mr. Trump is equally unmoved by any sense that to speak as a president is a kind of kabuki or performance art, in which one doesn’t so much talk as signal. He has learned that he can just show up and run his mouth, and he’ll be adored regardless.

Some suppose Mr. Trump started talking down deliberately in order to portray folksiness. But this imputes to him a sociological sensitivity, a reflective, outwardly focused theory of mind, that he shows no evidence of otherwise. More likely, Mr. Trump has simply taken the path of least resistance.

Note, for example, another difference between the younger Mr. Trump and today’s: comfort. In those old interviews he is a more buttoned-up person, obviously somewhat wary of the eyes upon him. Now, he revels in the attention as if he’s the paterfamilias at the head of a Thanksgiving dinner table. The articulate Mr. Trump of yore was wearing a linguistic tuxedo; today’s swivel-tongued Mr. Trump is in a linguistic track suit.

The difference between the younger man talking in sentences and the older one talking in vocal ejaculations is evidence not of decline but authenticity – he has settled into his normal. Late in life an artless man has learned that he could leave his linguistic fly unzipped and life would go on. It may not be pretty, but it isn't a sign that his pants are going to fall down.