This book both is and isn\'t a debut novel. Jennie Erdal has previously dabbled in fiction – but in the voice of, and under the name of, the larger-than-life publisher Naim Attallah, whose ghostwriter she was for 15 years. In her 2004 memoir Ghosting she described, with wry humour and a slightly disconcerting detachment, how he chivvied her into producing romantic thrillers brimming with, as he put it, plenty of \"fucky-fucky\", including orgasms simultaneously transmitted across different continents – \"but very distinguished\". With a title taken from David Hume and the subtitle \"A Philosophical Adventure\", the first novel to appear under her own name announces itself as a far more serious enterprise. As in her memoir, Erdal is still fascinated by disappearing acts. She was a translator before she became a ghost, and described both careers as a state of \"invisible presence\". Her narrator, Edgar Logan, is also a translator, delighting in a life that is \"low-profile to the point of invisibility\". Of French and Scottish parentage, Edgar has come to Edinburgh from Paris to work on a French edition of Hume\'s essays, flat-swapping with a Scottish academic writing a book on Sartre (\"no doubt in my mind as to who had got the better deal\"). While in Edinburgh he is sucked into the orbit of a charismatic, self-hating philosopher, Harry Sanderson, and his enigmatic artist wife, Carrie. Edgar has \"always worked on the principle that a translator is a guest in somebody else\'s house\"; as he listens to both versions of the breakdown of their marriage, he realises that he is a guest in Harry and Carrie\'s city, their language and their lives, and begins to wonder why he has never fully occupied his own. This quiet, controlled novel is more properly a philosophical mystery than an adventure. Several urgent mysteries are revealed over the course of the book – why Harry appears to be stalking Edgar; who is behind the ghost in Edgar\'s flat; how Edgar became the tentative, provisional man that he is – and several, beyond solution, are explored with supple vigour: the mysterious web of connections between translation and fiction, philosophy and life, art and interpretation. The missing shade of blue, which Erdal places at the centre of the book, refers to Hume\'s idea that although all our knowledge of the world comes through our senses, a man shown a series of graduating blues with a missing shade could intuit its colour from somewhere within himself. It\'s a fertile image through which to consider Erdal\'s themes: whether we can vault the limitations of our genes and our upbringing, how to address the perils of overthinking, whether love can be apprehended by the loveless. Carrie offers Edgar a similarly fascinating puzzle, with a story about a translator friend whose descriptions of her daughter\'s illness and recovery are irresistibly reminiscent of a Japanese novel she had previously translated. Has she been so very influenced by the book she worked on? Or did the English version of that novel contain more of her than it did of its own author? These are deep waters and dense themes, marshalled with a light touch and a dry wit. We learn a lot about the zen of fly fishing, the romance of bookselling and the merits of David Hume. Too much of the book, though, is aperçu-stuffed and relentlessly discursive, whether it\'s Edgar and Harry conducting boozy Socratic dialogues or Edgar and Carrie earnestly agreeing with each other about how ineffable and mysterious translation and painting are. The novel\'s riffs on fictionality, meanwhile, have become over-familiar. Harry – bitter, drunken, physically crumbling, railing against philosophy, academia, women and love – is the book\'s locus of energy, yet there\'s something off-centre about Edgar\'s fascination with him, the way he ventriloquises his monologues of despair. Something of both men is lost in the translation. If the unexamined life is not worth living, the examined life, all the characters agree, will \"get you into all sorts of trouble\". Erdal is good at trouble – calm, unflappable, unflinching – and an elegant, humane interpreter of the dangers and delights of philosophy. It can, as Carrie warns, \"seriously damage your health\".