THE thunder of historical revisionism, in the name of racial or sexual justice, just keeps on rolling and swelling. And you can’t predict what plinths it will start to fracture next. I’m fascinated by the impact this process has recently had on two robust patriarchs of Scottish history: the 19th-century naturalist John Muir, and the 18th-century philosopher David Hume.

This week in the US, the Sierra Club – the largest environmental organisation in the country – renounced and condemned the racist language of its founder, Muir.

Born and brought up in Dunbar, Muir roamed and recorded the wildernesses of America. His literary advocacy for nature was crucial to the establishment of the US’s great National Parks (such as the Grand Canyon, Yosemite and Sequoia).

Yet when he came upon struggling black and native American humans in these “pristine” places, Muir’s language could trade in the worst of stereotypes. In order to check these claims, I have a reference to a particular source. As a judge in the 2010 Scottish Books of the Year awards, I pushed strongly for the American academic Donald Worster’s biography of Muir to win (it did).

Via the joys of Kindle, I retrieved a copy the other day and did some keyword searching. And it’s all there. Take this from his Florida and Cuba Trip: “The negroes are very lazy [revised to “easy-going” in the published version, notes Worster] and merry … One energetic white man, working with a will, would easily pick as much cotton as half-a-dozen sambos and sallies”.

And this from My First Summer in the Sierra (1868), on encountering Indian tribes: “A strangely dirty and irregular life these dark-eyed, dark-haired half-happy savages lead in this clean wilderness … How glad I was to get away from the gray, grim crowd and see them vanish down the trail!

“Yet it seems sad to feel such desperate repulsion from one’s fellow beings, however degraded. To prefer the society of squirrels and wood chucks to that of our own species must surely be unnatural.”

The deep trouble here (as well as the visceral prejudice) is Muir’s implication that these groups were a stain on the “clean wilderness”. As many scholars have pointed out, the green meadows that allowed Muir perfect views of Yosemite were actually produced by First Nations cultivation of the land.

And at the time, Muir seems to be have been generally ignorant that these tribes had recently been subjected to the most vicious clearance from their lands, by white prospectors for gold, in the Sierra Nevada of the 1840s and 50s. Whatever “degradation” Muir saw in them was a spectacle produced by dispossession and violation.

The current Sierra Club executive director, Michael Brune, is clear-eyed about the consequences of Muir’s de-humanised vision of nature: “It’s the idea that exploring, enjoying, and protecting the outdoors can be separated from human affairs.

“Such wilful ignorance is what allows some people to shut their eyes to the reality that the wild places we love are also the ancestral homelands of Native peoples forced off their lands in the decades or centuries before they became national parks.”

Brune continues: “It allows them to overlook, too, the fact that only people insulated from systemic racism and brutality can afford to focus solely on preserving wilderness.”

Muir’s views evolved in his later years. He famously challenged the Indian exterminist Colonel Boyce, to his face, about such “mean and brutal policy”. But the company Muir kept on the early boards, circles and memberships of the Sierra Club, was noxious. It included white supremacists and eugenicists such as Joseph LeConte, Henry Fairfield Osborn and David Starr Jordan.

I do feel for organisations such as the John Muir Trust in Scotland. Their inspiration to defend wilderness in Scotland is founded in gritty protests against the MoD, over their plans for Knoydart’s forests. It’s certainly not about any of Muir’s misanthropic (and worse) fantasies.

BUT a different kind of thunder is rolling down the mountains at the moment. And it comes from a generation, in all its diversity, that won’t let the storm of historical justice abate. We all have to battle, honestly and openly, through the consequences of this.

The protests about David Hume a few weeks ago – specifically, a petition urging an Edinburgh University tower in his name be retitled – have their own intriguing dynamics. As many commentators have noted, out of all the Scottish Enlightenment philosophers, Hume has often been cited as the one with the greatest contemporary relevance.

Hume anticipated neuroscience’s inquiry into how “reason is the slave of the passions”. From this, he appeals to modern religious sceptics and anti-populists, who urge their fellow citizens to wake up to how emotionally driven (and thus partial) their “truths” are.

In a raging age of culture wars of all kinds, Hume’s pragmatism and relativism feels to many like a useful box of mental tools. But again, as with Muir, Hume’s archive can’t lie. There it is, in his 1753 essay On National Characters: “I am apt to suspect that the negroes and in general all of the other species of men (for there are four or five different kinds) to be naturally inferior to the whites.

“There never was a civilised nation of any other complexion than white, nor even any individual eminent either in action or speculation. No ingenious manufactures amongst them, no arts, no sciences …”

That’s enough; it gets worse as it goes on. The Cambridge historian Felix Waldmann has discovered in his researches that Hume “advised his patron Lord Hertford to buy a slave plantation, facilitated the deal and lent £400 to one of the principal investors”. And when criticised for racism in 1770 by James Beattie of Aberdeen, Hume was “unmoved”, leaving the offending essay unedited in later collections.

The questions circulating around this are huge. Must we re-examine the Scottish Enlightenment for its commitment to “progress” and “improvement”, if those notions seem to imply a racial hierarchy (ie “worse” and “better” races)? And grimmer still, if they then serve to provide a rational-scientific justification for it?

As for Hume, in particular, you wonder whether a philosopher who elevated the “passions” so much was simply contented with his various prejudices. He happily allowed them to shake his palace of reason.

Is there any way up or out here? One of Hume’s most unexpected fans (“my favourite philosopher in the English language”) is the leading black American intellectual Cornel West. In a 2011 interview, the Professor was perfectly aware that Hume is “one of the major propagandists and defenders of white supremacy”.

But West seems to value “the sheer quality of Hume’s mind and the subtlety of his philosophic intelligence”, particularly for its sceptical challenge to West’s Christian religious faith (“I have to read him every year because it makes me stronger”).

Like reading Martin Heidegger, the Nazi-supporting German philosopher “who taught me too much”, West knows reading Hume puts him “in close conversation and sometimes in very close intellectual proximity to gangsters … So you find yourself having to be jazzlike: protean, flexible, fluid, open-minded”.

Do we have the space, and patience, to behave in West’s “jazzlike” way – riffing alongside the toxic elements of enlightened “improvers” such as John Muir and David Hume, in their respective fields?

West’s great wisdom and capacity – while he also stands on the frontlines of the most urgent struggles for justice and equality – is a high bar for any of us to get over.

Yet when a storm abates (knowing there will always be another), there is ozone in the air. City streets are cleaned of their grime and filth. The clear blue breaks through.

Then there might be clarified conditions under which we value Hume the sceptic, and Muir the naturalist. But still much rough weather yet to come, I’d say.