On a typical day, Denis Diderot might write about ancient Chinese music in the morning, study the mechanics of a cotton mill in the afternoon, then work on a play after dinner.

Diderot and the Art of Thinking Freely, by Andrew S Curran. Photograph: Other Press

The man was both workhorse and Enlightenment polymath: a genius who wrote 7,000 articles for his celebrated Encyclopédie while producing revolutionary novels, groundbreaking art criticism and proto-Darwinian science fiction. To round this out, he also authored some of the century’s most influential political writing – much of it now eerily prescient in the Trump era.

As a young man, Diderot did not concern himself with politics per se. His target was the Catholic church and organized religion. If Voltaire became the era’s most prominent anti-cleric, Diderot was its most ingenious and freethinking atheist. In the summer of 1749, he published a 90-page book proclaiming that the Christian God was little more than a nefarious figment of people’s imaginations. It earned him a three-month stay in prison.

His career as a political writer began in earnest after he spent several months in St Petersburg, visiting Catherine the Great. He made this arduous 2,000-mile journey to pay tribute to a woman he considered Europe’s most enlightened monarch. But he had another goal: convincing Catherine to inch her country toward a more representative government.

Beware the corruption of morals and the scorn of laws; beware of an unbalanced distribution of wealth Denis Diderot

Diderot generally spent three afternoons a week in Catherine’s “Little Hermitage”, the three-storey neoclassical townhouse adjacent to the 700,000 sq ft Winter Palace.

Catherine often did needlework while Diderot inveighed against religious persecution, bigotry and superstition. But he also brought up touchy political topics: the unhealthy concentration of wealth among the Russian aristocracy, the plight of millions of serfs. He added concrete recommendations: Catherine should invest in public schools and universities and hire teachers to instruct young girls in human sexuality. He even recommended the empress move her capital to Moscow, since having a “capital at the end of an empire” was like having an animal whose “stomach was at the end of its big toe”.

The Marble Room, in the Winter Palace in St Petersburg. Photograph: Alamy Stock Photo

For a time, Diderot believed that the empress might be the rarest of all creatures: a sitting monarch willing to let a philosopher help liberate her country from medieval institutions and prejudices. Two months later, he asked why she was not taking his advice. She told him, kindly but firmly, that his political ideas would do well in books but badly in practice. Unlike her, he worked on paper. She was obliged to work on human skin.

Before his coach rolled out of St Petersburg, Diderot promised Catherine he would never publish anything negative about her. He kept his word. He had, however, soured on the idea of reforming the continent’s “enlightened despots”.

Upon his return to France, Diderot produced a series of reflections on the Russian empire that, per his will, were sent to the empress after his death in 1784. Catherine was livid: Diderot not only called her a despot, he made very clear that the Russian people had the inalienable right to condemn a tyrannical ruler to death. As demonstrations of popular sovereignty went, he could hardly have been more clear.

Diderot’s musings on Russia dovetailed with a larger set of writings, most of which were published without his name in one of the most influential books of the 18th century, the Philosophical and Political History of the Two Indies. In the 1770s and 1780s, everybody from future French revolutionary Georges Danton to Benjamin Franklin pored over this sweeping survey of world history and colonization.

Diderot, who was responsible for about 20% of the book, was its most daring ghostwriter. In what amounted to a moral tour of the globe, he castigated European expansion, indicted the slave trade and argued for the rights of indigenous peoples. In one memorable passage, he predicted the rise of a black Spartacus in the Caribbean who would put white planters to the sword. This was more than a decade before Toussaint L’Ouverture turned a slave rebellion into a revolution on the island of Saint-Domingue, present-day Haiti.

Diderot also turned his attention to France, nine years before the revolution. Addressing Louis XVI, he warned that the empire had become a powder keg with two classes of citizens: one which “wallows in its wealth and flaunts its luxury”, the other an impoverished underclass that was becoming justifiably “indignant”.

The atrium, Trump Tower. Photograph: KFS//Rex/Shutterstock

But the part of the world that fascinated Diderot most was the fledgling United States of America. An unabashed supporter of American independence, he reveled in the birth of a federation of independent states that, in his view, combined the “freedom of democracy” with the “political strength of a monarchy”. If this new republic were only able to rid itself of human bondage, he believed, it might actually become the Promised Land.

Diderot regretted being too old to travel to this “land of tolerance, morals, laws, virtue and liberty”. But he did not let this deter him from giving advice to the “American Insurgents”, as he called them. Repeatedly urging the revolutionaries to lay down their lives before sacrificing the smallest part of their liberty, he also cautioned them to avoid the mistakes that had plagued Europe for centuries:

People of North America, may the example of all those nations that have preceded you, and especially that of your motherland, instruct you. Beware of the affluence of gold that brings with it the corruption of morals and the scorn of laws; beware of an unbalanced distribution of wealth that will give rise to a small number of opulent citizens and a horde of citizens in poverty, a situation that will engender the insolence of some and the deprivation of others.

The real threat to American democracy, Diderot believed, would not come from foreign powers: it would stem from within, from unbridled luxury, class tensions, political corruption and, in the worst scenario, from the rise of a cynical autocrat who turns his back on the founding values of the Republic.

As age-old prophecies go, this vision of the United States is as far-sighted as it is disheartening.