The Voices Within: The History and Science of How We Talk to Ourselves By Charles Fernyhough Profile Books. 2016. £16.99 (hb). 352 pp. ISBN 9781781252796

This is a truly exceptional book for its scope, richness of detail and originality. It is about inner speech, a subject that is central to our understanding of the nature, phenomenology and origins of auditory verbal hallucinations. Fernyhough says of his goal: ‘I want to ask what it is like to have this sort of thing going on in your head [inner speech]. I want to investigate how it feels to be caught up in the flow of impressions, ideas and internal utterances that make up our stream of consciousness’. And he does just that. Furthermore, he persuades us (if we ever needed persuading) that ‘Talking to ourselves is a part of human experience which, although by no means universal, seems to play many different roles in our mental lives’.

I was surprised to discover that less than a quarter of us engage in inner speech. But, it was not much of a surprise that the four distinct categories of inner speech are the faithful friend, the ambivalent parent, the proud rival and the calm optimist. The function of inner speech at the very least includes the regulation of action and the capacity to take different perspectives. There appears to be a developmental dimension to the role of inner speech in regulating behaviour. Fernyhough relies on Vygotksy's theory on the transition of social speech to private speech (that is speech that acts as a tool to assist a child in performing particular tasks), and ultimately into inner speech. Accompanying this transition is a significant transformation in the form of utterances such that they become abbreviated, even truncated, but yet retaining a dialogic aspect.

Fernyhough is at his very best when dealing with inner speech. The interest from a psychiatrist's point of view lies in the fact that auditory verbal hallucinations are currently conceived of as inner speech in which there is a misattribution error leading the person to locate the experience as deriving from external space. In this sense, a science of inner speech will demonstrably shed light on the nature and origins of auditory verbal hallucinations. Fernyhough's implicit approach is to normalise auditory verbal hallucinations by drawing continuities with self-talk in sportsmen, the nature of silent reading, and verbal hallucinatory experiences of writers. The problem is that there is not enough recognition that analogy, the correspondence in certain respects between things, is not evidence of identity. It is like arguing that normative blinking and blinking tics are on a continuum or that arm gestures and hemiballismus are continuous entities on account of superficial family resemblance.

This is a book that informs as well as provoking thought and reflection. It could say more about the phenomenology of auditory verbal hallucinations. Although it ignores the continuities between such experiences as thought echo, thought broadcasting and auditory verbal hallucinations, it addresses the potential neurology of auditory verbal hallucinations. It is quite simply a remarkable book.