“Opposing reason is, by definition, unreasonable.” Steven Pinker is fond of definitions. Early on in this monumental apologia for a currently fashionable version of Enlightenment thinking, he writes: “To take something on faith means to believe it without good reason, so by definition a faith in the existence of supernatural entities clashes with reason.” Well, it’s good to have that settled once and for all. There is no need to trouble yourself with the arguments of historians, anthropologists and evolutionary biologists, who treat religion as a highly complex phenomenon, serving a variety of human needs. All you need do is consult a dictionary, and you will find that religion is – by definition – irrational.

Similarly, you don’t need to bother about what the Enlightenment was actually like. By any standards, David Hume was one of the greatest Enlightenment thinkers. It was the sceptical Scottish philosopher who stirred Immanuel Kant – whose well-known essay on Enlightenment Pinker quotes reverently at the start of the book – from what Kant described as his “dogmatic slumber”. Pinker barely mentions Hume, and the omission is not accidental. He tell us that the Enlightenment is defined by a “non-negotiable” commitment to reason.

Yet in A Treatise of Human Nature (1738), Hume wrote: “Reason is, and ought only to be, the slave of the passions and can never pretend to any other office than to serve and obey them.” Hume believed being reasonable meant accepting the limits of reason, and so too, in quite different ways, did later Enlightenment rationalists such as Keynes and Freud. Pinker’s Enlightenment has little in common with the much more interesting intellectual movement that historically existed.

One of the consequences of this unhistorical approach is that Pinker repeats fallacies that have been exposed time and time again. He is an evangelist for science – or, to be more exact, an ideology of scientism. Along with reason, humanism and progress, science features as one of the core Enlightenment values that Pinker lists at the start of the book. But for him science is more than a bunch of methods that are useful in conjecturing how the world works: it provides the basis of ethics and politics.

He summarises this claim in a formula: “Entro, evo, info. These concepts define the narrative of human progress, the tragedy we were born into, and our means of eking out a better existence.” Here, “entro” denotes entropy, the process of increasing disorder that is identified in the second law of thermodynamics. “Evo” refers to the evolution of living organisms, which absorb energy and thereby resist entropy. “Info” is information, which when collected and processed in the nervous systems of these organisms enables them to wage their war against entropy.

For Pinker, the second law of thermodynamics doesn’t simply identify a universal regularity in the natural world, “it defines the fate of the universe and the ultimate purpose of life, mind, and human striving: to deploy energy and knowledge to fight back the tide of entropy and carve out refuges of beneficial order”.

There is nothing novel in scientism. The Victorian prophet of social evolution, Herbert Spencer, believed that the universe, life and society were moving from undifferentiated simplicity to a higher state of complex order. In politics, this meant a movement towards laissez-faire capitalism. In social contexts, “survival of the fittest” – an expression Spencer invented after reading Darwin’s On the Origin of Species – meant that anyone unable to stay afloat in such a society would struggle, sink and then disappear. Spencer welcomed this process, since for him it was evolution in action – the movement from lower to higher forms of life.

Pinker is an ardent enthusiast for free-market capitalism, which he believes produced most of the advance in living standards over the past few centuries. Unlike Spencer, he seems ready to accept that some provision should be made for those who have been left behind. Why he makes this concession is unclear. Nothing is said about human kindness, or fairness, in his formula. Indeed, the logic of his dictum points the other way.

Many early-20th-century Enlightenment thinkers supported eugenic policies because they believed “improving the quality of the population” – weeding out human beings they deemed unproductive or undesirable – would accelerate the course of human evolution. When Pinker touches on eugenics in a couple of paragraphs towards the end of the book, he blames it on socialism: “The most decisive repudiation of eugenics invokes classical liberal and libertarian principles: government is not an omnipotent ruler over human existence but an institution with circumscribed powers, and perfecting the genetic make-up of the human species is not among them.” But a theory of entropy provides no reason for limiting the powers of government any more than for helping the weak. Science cannot underwrite any political project, classical liberal or otherwise, because science cannot dictate human values.

Exponents of scientism in the past have used it to promote Fabian socialism, Marxism-Leninism, Nazism and more interventionist varieties of liberalism. In doing so, they were invoking the authority of science to legitimise the values of their time and place. Deploying his cod-scientific formula to bolster market liberalism, Pinker does the same. Scientism is one of the Enlightenment’s bad ideas. But bad ideas do not evolve into better ones. They keep on recurring, often in cruder and sillier forms than in the past. Pinker’s formula for human progress is a contemporary example.

To be sure, for Pinker there are no bad Enlightenment ideas. One of the features of the comic-book history of the Enlightenment he presents is that it is innocent of all evil. Accordingly, when despots such as Lenin repeatedly asserted that they engaged in mass killing in order to realise an Enlightenment project – in Lenin’s case, a more far-reaching version of the Jacobin project of re-educating society by the methodical use of terror – they must have been deluded or lying. How could a philosophy of reason possibly be implicated in murderous totalitarianism? Like the faithful who tell you Christianity is “a religion of love” that had nothing to do with the Inquisition, Pinker stipulates that the Enlightenment, by definition, is intrinsically liberal. Modern tyrannies must therefore be products of counter-Enlightenment ideologies – Romanticism, nationalism and the like. Enabling liberals to avoid asking difficult questions about why their values are in retreat, this is a popular view. Assessed in terms of historical evidence, it is also a myth.

Many Enlightenment thinkers have been avowedly or implicitly hostile to liberalism. One of the most influential, the 19th-century French positivist Auguste Comte – not discussed by Pinker – promoted a brand of scientism that was overtly anti-liberal. Human progress meant following the path of reason and moving from magical thinking to scientific inquiry. In a society based on science there will be no need for liberal values, since moral and political questions will be answered by experts.

Comte admired the Middle Ages as a time when society was healthily “organic” and unified by a single orthodoxy; but the organic society of the future would be ruled by science, not monotheism. The superstitious faith of earlier times would be supplanted by what he called “the Religion of Humanity” – a rationalist creed in which an imaginary version of the human species would occupy the place of the Supreme Being. Comte’s core ideas – reason, science, progress and humanism – are precisely those that Pinker lists at the start of this book as the central values of the Enlightenment. Interestingly, neither of them mentions freedom or toleration.

The link between the Enlightenment and liberal values, which Pinker and many others today assert as a universal truth, is actually rather tenuous. It is strongest in Enlightenment thinkers who were wedded to monotheism, such as Locke and indeed Kant. The more hostile the Enlightenment has been to monotheism, the more illiberal it has been. Comte’s anti-liberalism inspired Charles Maurras, a French collaborator with Nazism and the leading theorist of Action Française – a fascistic movement formed during the Dreyfus affair – in his defence of integral nationalism. Lenin continued the Jacobins’ campaign against religion as well as their pedagogy of terror.

Instead of acknowledging that the Enlightenment itself has often been illiberal, Pinker presents a Manichean vision in which “Enlightenment liberal values” are besieged on every side by dark forces. Amusingly, he is in no doubt as to the identity of the intellectual master-criminal behind this assault. The Professor Moriarty of modern irrationalism, the “enemy of humanism, the ideology behind resurgent authoritarianism, nationalism, populism, reactionary thinking, even fascism” can at last be revealed:

If one wanted to single out a thinker who represented the opposite of humanism (indeed of pretty much every argument in this book) one couldn’t do better than the German philologist Friedrich Nietzsche. Nietzsche helped to inspire the romantic militarism that led to the First World War and the fascism that led to the Second. The connections between Nietzsche’s ideas and the megadeath movements of the 20th century are obvious enough; a glorification of violence and power, an eagerness to raze the institutions of liberal democracy, a contempt for most of humanity, and a stone-hearted indifference to human life.

Searching for some intellectual authority for this wild diatribe, Pinker cites Bertrand Russell’s History of Western Philosophy, where Russell denounced Nietzsche as a Romantic enemy of reason who preached a life of instinct and emotion. Published immediately after the end of the Second World War, Russell’s assessment of Nietzsche was understandably crude. Today it would not pass muster in a first-year undergraduate’s essay.

A lifelong admirer of Voltaire, Nietzsche was a critic of the Enlightenment because he belonged in it. Far from being an enemy of humanism, he promoted humanism in the most radical form. In future, humankind would fashion its values and shape its destiny by its own unfettered will. True, he conferred this privilege only on a select few.

He recognised no principle of human equality. But where does concern with equality come from? Not from science, which can be used to promote many values. As Nietzsche never tired of pointing out, the ideal of equality is an inheritance from Judaism and Christianity. His hatred of equality is one reason he was such a vehement atheist.

The message of Pinker’s book is that the Enlightenment produced all of the progress of the modern era and none of its crimes. This is why he tries to explain 20th-century megadeaths by reference to Nietzsche’s supposedly anti-Enlightenment philosophy. Here he has shifted his ground. In The Better Angels of Our Nature (2011), Pinker represented the Hemoclysm – a term referring to the early 20th-century spasm of mass killing, which he uses to lump together the two world wars, the Soviet Gulag and the Holocaust – as not much more than a statistical fluke. What explains this change of view? Pinker cites no change in the historical evidence that is available on the subject.

Instead, there has been a shift in the mood of liberals. Less than a decade ago, they were confident that progress was ongoing. No doubt there would be periods of regression; we might be in one of those periods at the present time. Yet over the long haul of history, there could be no doubt that the forces of reason would continue to advance. Today, liberals have lost that always rather incredible faith. Faced with the political reversals of the past few years and the onward march of authoritarianism, they find their view of the world crumbling away. What they need at the present time, more than anything else, is some kind of intellectual anodyne that can soothe their nerves, still their doubts and stave off panic.

This is where Pinker comes in. Enlightenment Now is a rationalist sermon delivered to a congregation of wavering souls. To think of the book as any kind of scholarly exercise is a category mistake. Much of its more than 500 pages consists of figures aiming to show the progress that has been made under the aegis of Enlightenment ideals. Of course, these figures settle nothing. Like Pinker’s celebrated assertion that the world is becoming ever more peaceful – the statistical basis of which has been demolished by Nassim Nicholas Taleb – everything depends on what is included in them and how they are interpreted.

Are the millions incarcerated in the vast American prison system and the millions more who live under parole included in the calculus that says human freedom is increasing? If we are to congratulate ourselves on being less cruel to animals, how much weight should be given to the uncounted numbers that suffer in factory farming and hideous medical experiments – neither of which were practised on any comparable scale in the past?

It would be idle to pursue such questions. The purpose of Pinker’s laborious graphs and figures is to reassure his audience that they are on “the right side of history”. For many, no doubt, the exercise will be successful. But nagging questions will surely return. If an Enlightenment project survives, what reason is there for thinking it will be embodied in liberal democracy? What if the Enlightenment’s future is not in the liberal West, now almost ungovernable as a result of the culture wars in which it is mired, but Xi Jinping’s China, where an altogether tougher breed of rationalist is in charge? It is a prospect that Voltaire, Jeremy Bentham and other exponents of enlightened despotism would have heartily welcomed.

Judged as a contribution to thought, Enlightenment Now is embarrassingly feeble. With its primitive scientism and manga-style history of ideas, the book is a parody of Enlightenment thinking at its crudest. A more intellectually inquiring author would have conveyed something of the Enlightenment’s richness and diversity. Yet even if Pinker was capable of providing it, intellectual inquiry is not what his anxious flock demands. Only an anodyne, mythical Enlightenment can give them what they crave, which is relief from painful doubt.

Given this overriding emotional imperative, presenting them with the actual, conflict-ridden, often illiberal Enlightenment would be – by definition, one might say – unreasonable. Judged as a therapeutic manual for rattled rationalists, Enlightenment Now is a highly topical and much-needed book. In the end, after all, reason is only the slave of the passions.

John Gray’s new book, “Seven Types of Atheism” will be published in April by Allen Lane.

Enlightenment Now: the Case for Reason, Science, Humanism and Progress

Steven Pinker

Allen Lane, 576pp, £25