“Is Life Worth Living?” was the title of a lecture given in 1896 by philosopher and psychologist William James. It began by mentioning a similarly-entitled self-help book—to which, when it had been published 15 years earlier, newspapers had proposed the facetiously ambiguous answer: “That depends on the liver.”

He himself, James said, “cannot be jocose” about the question which had been a vital and serious one for him during his early 20s when he had been contemplating suicide. Waking every morning “with a horrible dread at the pit of my stomach”, and with a sense of everything’s ultimate precariousness, he had wondered “how other people could live, how I myself had ever lived, so unconscious of that pit of insecurity beneath the surface of life”. To this, “the morbid-minded way, as we might call it [of viewing life], healthy-mindedness pure and simple seems unspeakably blind and shallow”.

In the end, however, James seems to have decided deliberately to adopt what he had mocked—“our usual refined optimisms and intellectual and moral consolations”— and to act as if life were indeed worth living, all the more intensely because of its terrifying undertow.

John Kaag, a philosophy professor at the University of Massachusetts, was, he says, inspired to write this book after arriving at the scene of a suicide on a freezing February day in 2014. A young man had jumped from the 15th floor of the William James Hall at Harvard University just after Kaag had cycled past it. Standing with other onlookers at the edge of the cordoned-off area, Kaag felt that they should be asking “not ‘What had happened?’ but rather ‘Why did it happen?”’

He wrote Sick Souls, Healthy Minds: How William James Can Save Your Life “for the fellow sick-souled”, to show that they “have a companion in misery: James”, and to “explore the ‘maybe’ of life’s worth”. It intersperses an unsystematic biography of William James with Kaag’s reflections about his own life, and with self-help-promoting wisdom.

Kaag has already written two popular philosophy books of the same hybrid sort, American Philosophy: A Love Story, and Hiking With Nietzsche: On Being Who You Are. James, like Nietzsche, would seem a very suitable protagonist. Both philosophised viscerally, their philosophy being spun out of intense, anguished but exultant living. Each exemplified Nietzsche’s famous remark that whereas the philologist or chemist or specialist in fungi can keep their expertise apart from the rest of their lives, “in the philosopher there is nothing impersonal”. Like Nietzsche a perpetual invalid, James embodied Nietzsche’s claim that “being sick can be an energetic stimulant to life, to more life”. Despite, or probably because of, his depressiveness, he was always striving to feel “the zest, the tingle, the excitement of reality”. “Live hard!” he urged his adolescent son, just as Nietzsche exhorted “Live dangerously!” “Be not afraid of life”, he urged members of Harvard’s Young Men’s Christian Association, in the lecture “Is Life Worth Living?” in 1896. “Believe that life is worth living, and your belief will help create the fact.”

“Such a philosophy,” writes Kaag of James’s:

would be wholly unnecessary were it not for the fact that many of us seem to teeter on the brink of the abyss. In 2010, I was there myself. I was 30, in the midst of a divorce, and had just watched my estranged, alcoholic father die. And I was at Harvard on a postdoc writing about—you guessed it—William James.

I wanted and expected to be charmed by this book, but the trouble with Kaag’s choice of such an extreme and emotional subject as James is that interlacing his own life and thoughts make him seem merely pallid by comparison. Quoting so magnificent a stylist only emphasises the gap between subject and author, and anyway Kaag doesn’t quote him enough. He doesn’t do justice to James, leaving out James’s theory of radical empiricism, his “anti-intellectualism”, even (symptomatically) the constant excitable italicisation of his writing. The mysticism that James mingled with his hard-edged pragmatism is reduced to a dabbling in séances.

William James was born in New York City in 1842, brother to the novelist Henry, and eldest son of an eccentric man (also Henry) of private means who belonged to the recherché Christian sect of Swedenborgianism. William and his four siblings were educated by travelling all over Europe, attending schools in Paris, Boulogne, Geneva, and Bonn, as well as on Rhode Island. They were given a great deal of freedom. Having wanted to be an artist and taken intensive lessons in painting, William decided to study chemistry, then biology and finally medicine, at Harvard; though he postponed medical studies for a year in order to go on a scientific expedition to the Amazon organised by Louis Agassiz, a then-preeminent zoologist and geologist. Sea-sick and wretched, James had an excuse to return to Boston after a minor bout of smallpox.

He was always prone to eye problems, and to headaches, digestive disorders and undiagnosed back pain, which often prevented him from walking or even sitting upright—all symptoms which, as his mother constantly suggested, might have been psychosomatic. He was morbidly depressed. Exacerbating his sense of powerlessness was the dread that the theory of determinism might be true: that humans, like everything else, are subject to cause and effect, and that our apparent power to choose what we do is merely an illusion. Darwin’s recently published On the Origin of Species seemed to outlaw the notion of free will from any worldview. Reality must surely consist of inexorable chains of cause and effect, yet also be terrifyingly unpredictable, dependent on the haphazard fragility of survival.

But on April 30 1870, James wrote in his diary:

I think that yesterday was a crisis in my life. I finished the first part of Renouvier’s second Essai and see no reason why his definition of free will—“the sustaining of a thought because I choose to when I might have other thoughts”—need be the definition of an illusion. At any rate, I will assume for the present—until next year—that it is no illusion. My first act of free will shall be to believe in free will.

James’s salvation from despair and suicide was not, as he would have acknowledged, directly caused by reading an unknown French philosopher; Renouvier’s argument is anyway dubious. But “pure reason”, as James said, is rarely what settles our opinions; and “facts have hardly anything to do with making us either determinists or indeterminists”.

One of the things for which James is most famous is his development of the idea of pragmatism (originated by another American, Charles Peirce)—that truth is not statically there to be perceived or discovered but is, in many cases, what we create in the stride of living. We make it the case that we can jump across an abyss by actually jumping across it—or we fail to, and fall in. James (as Kaag doesn’t say) derided what he called “intellectualism”, the belief that our minds encounter “a world complete in itself”, and need simply to ascertain its contents while having no power to re-determine its already-given character. Instead of there being a correspondence between our minds and reality, James said, there is “a rich and active commerce”. We “marry” new ideas to old opinions, otherwise they would make no sense to us. Beliefs can be considered hypotheses, and the deadness or liveness of a hypothesis “are not intrinsic properties but relations to the individual thinker . . . measured by his willingness to act”. We can change the world through our actions, and create a new belief in what will by then be true. By making the value of life a “live hypothesis”, we make our lives valuable.

Kaag does not sufficiently explain James’s pragmatism nor the way his stringent cheerfulness was achieved by almost management-type skills. The diary entry stating that he would assume for the present, for a year at any rate, that he had free will, was a statement of policy. He was determined (maybe in both senses) to cheat the inexorability of cause and effect by steadfastly cultivating useful habits, thereby to “accumulate grain on grain of willful [sic] choice like a very miser”. He went on to complete his medical degree, co-found the department of experimental physiology (equipped with laboratories and testing instruments) at Harvard Medical School, teach physiology and psychology at Harvard University, write a two-volume book on psychology (published in 1890, having taken 12 strenuous years to complete), write the extraordinary The Varieties of Religious Experience and many lectures and essays (subsequently collected), and marry and have five children. Judging from letters and diaries, his wife was a saint.

Kaag quotes approvingly James’ suggestion as to the pragmatics of love—that showing unmerited faith in the existence of someone else’s amatory interest not only increases but often evokes it in the first place: simply waiting for objective evidence of it would probably pre-empt its ever happening. But Kaag also suggests the hubristic dangers of pragmatism. When he was having doubts on the morning of his (first) wedding, his mother urged him to go ahead—he could make it true that his marriage would be happy. But he couldn’t—or it seems as if he couldn’t, just as it might have seemed, to the believer in or Jamesian creator of, free will, that he could.

We are given a lamentably unmoving description of Kaag sitting in the car with his daughter on his lap, weeping into her hair, during his father’s funeral. He was, as James surely would have been, immersed in self-observation as he wept, but he cannot convey this emotion (or perhaps even have it) as powerfully as James would have done. James, surely influenced by his own passionate nature, ridiculed the idea that emotions could simply be states of mind which cause us to have visceral sensations. Try to imagine having an emotion without having sensations (he challenged his readers)—being angry in a whisper and a slouched position, for instance—and what you imagine would be so “pale, colourless, destitute of emotional warmth” as not to be an emotion at all, merely a “feelingless cognition”.

As in the case of truth and belief, he reversed the accepted view. Emotions, he argued are “constituted by, and made up of, those bodily changes which we ordinarily call their expression or consequence.” Being sad is not the cause of crying, but is what it feels like to cry. Kaag seems to miss this essential point, but rightly diagnoses that James anticipates the notion of “biofeedback”. For if “we don’t cry because we’re sad, but are sad because we cry”, then, equally, stopping ourselves from crying (or slouching or raging) should diminish or even pre-empt the emotion we might have had. This sort of “whistle-a-happy-tune” self-help is what James advocated, and tried, if not always successfully, to practise himself.

“Is life worth living?” James’s answer is essentially the same as the “jocose journalists”, including the ambiguity. But it is uttered in a different tone of voice, in prose which can be compared (it often is) with that of brother Henry’s. That makes all the difference, and is what Sick Souls, Healthy Minds cannot properly capture. And if, as Nietzsche famously said, every great philosophy is “a confession on the part of its author and a kind of involuntary and unconscious memoir”, maybe the emphasis should be on “involuntary and unconscious”; maybe in philosophising the personal should be inadvertent.

Sick Souls, Healthy Minds: How William James Can Save Your Life

By John Kaag

Princeton, 224pp, £18.99