“What shall I say about this book? Why do I like it?” This, were I to put into words what passed through my mind, was roughly what I said to myself before beginning to write this review. Hovering somewhere in the space between these thoughts, another voice, inaudible to anyone but myself, asked: “How about coffee? Or maybe wine? Yes? No, better stick to coffee.” These words, however, do not quite convey the actual experience, which was altogether a mistier and less definitive one. This kind of inner conversation, though common enough, is not well documented or understood. The psychologist Charles Fernyhough, who became interested in the manifold ways in which we commune with ourselves, decided to investigate the phenomenon and his book, The Voices Within, is the intriguing result of his research.

The book explores a wide range of types of voice, from the everyday, such as my own rather banal example, to the creative and the bizarre. Voices are associated in the popular mind with schizophrenia, but they are also frequent attenders on other psychiatric disorders. During the years I spent working as a psychoanalyst, I became acquainted with many kinds of inner voice: nags, down-putters, savage persecutors, prophets of doom, the siren calls of idleness, the seductive beckonings of recklessness – these and many other soundtracks afflict people who are by no means mad but nonetheless are victims of vocal inner correspondents prejudicial to their health and balance.

Fernyhough begins by exploring current efforts to unpick everyday introspections. “Descriptive experience sampling” employs a method that allegedly trains participants to analyse their own inner communications. A device is worn that bleeps at random intervals, at which point the wearer is enjoined to take note of whatever has passed across her mind at the time. This is repeated until six such experiences have been noted. The subject is then interviewed about the nature of the inner response and its exact relation to spoken language. Over time, these interviews elicit progressively more detailed information, though Fernyhough is candid about the method’s shortcomings. The presuppositions of the interviewer are hard to factor out, for example.

Less controversial is the cultivation of inner speech to affect action. The 20th-century French psychologist and pharmacist Émile Coué became popular with “cure” by autosuggestion. Coué’s ‘“every day, in every way, I am getting better and better” is a maxim typical of the kind of encouragement advocated by sports coaches. In one enlightening study darts players were asked to aim at the board while inwardly voicing either positive or negative inner suggestions (“You can do it” or “you’re bound to miss”). Those who spoke positively to themselves scored consistently higher and it has long been noted that high-achieving athletes will talk more to themselves than their less successful rivals.

Simple encouragement is one thing but creative thought appears to depend on inner dialogue and the most generative dialogue takes the form of question and answer. The physicist Richard Feynman is quoted describing how in attempting to solve a scientific problem he conducts an internal interrogation. “The integral will be larger than this sum of the terms, so that would make the pressure higher, you see. No, you’re crazy. No, I’m not. No, I’m not.”

It is as if we are eavesdropping on a 'praying mind', where a conversation with God is acted out internally

Fernyhough suggests that the schizophrenic’s hallucinatory voices are a special case of this splitting of one’s own mind, so that an alternative voice becomes estranged from the self and then embodied as an external presence. His own view is that inner speech is best defined by what he calls dialogicality – an unappealing term for what does seem to be a sensible model for self-communion: a dialogue between different inner voices. We are all a crowd and it makes sense to assume that different aspects of ourselves will adopt different communication patterns. (It has long been my view that this is why it is so hard to keep new year’s resolutions, as the self who makes the resolution will only represent a sliver of the complex whole of the personality; the same can be said, unfortunately, for promises.)

The medieval mystic’s encounter with God or, as in the famous case of Joan of Arc, with angels is in Fernyhough’s view another, more elevated version of this inner dialogue. He considers the cases of Margery Kempe and Julian of Norwich, two very different women who both had transforming visions of a Christ who addressed them in person. Fernyhough is dismissive of the modern tendency to perceive these encounters as evidence of psychiatric disorder and, rightly, places the accounts within the ethos of a time when God’s presence was more salient than it is today. It is as if, he suggests, we are eavesdropping on a “praying mind”, where a conversation with God is acted out internally. In an important structural sense, this is no different from the schizophrenic’s voices: what is different is the quality of the voice. Fernyhough cites important work by the Dutch psychiatrist Marius Romme, whose thesis is that the voices heard by psychotics are messengers “communicating important information about unresolved emotional problems”.

“Messenger” is the correct translation of the Greek “angelos” or angel and if Fernyhough is to be believed, there is a sense in which we are visited all the time by good or bad angels and it is the ability to question and discriminate that distinguishes creative thoughtfulness from madness.

Salley Vickers’s new novel, Cousins, will be published by Viking in November. The Voices Within is published by the Wellcome Collection and Profile (£16.99). Click here to buy it for £12.99