Green's parents were both primary school teachers, who together authored a series of geography textbooks which became known as The Green Geographies.
She was educated first at the Ursuline Convent in Ilford, and later at the Woodford High School for Girls, a state school. In a book, Letters from Exile, she compared these two schools and made conclusions that preferred parentally financed to state education. She won the Senior Open Scholarship to Somerville College, Oxford aged 17.
In 1960 she was awarded a B.Litt. degree from Oxford University's faculty of Literae Humaniores (Philosophy), for a thesis, supervised by H. H. Price, entitled An Enquiry into Some States of Consciousness and their Physiological Foundation. From 1957 to 1960, Green held the post of Research Officer at the Society for Psychical Research in London. In 1961, Green founded and became the Director of the Institute of Psychophysical Research. The Institute's areas of interest were initially listed as philosophy, psychology, theoretical physics, and ESP. However, its principal work during the sixties and seventies concerned hallucinations and other quasi-perceptual experiences in normal subjects. Its main benefactor, from 1963 to 1970, was Cecil Harmsworth King, then Chairman of the IPC group, which owned the Daily Mirror.
In 1996 Green was awarded a DPhil degree by the Oxford faculty of Literae Humaniores for a thesis on causation and the mind-body problem. Green is an Honorary Research Fellow at the Department of Philosophy, University of Liverpool.
Green's basic philosophical position may be described as one of radical scepticism, based on a perception of what she calls 'the total uncertainty'. This perception leads her to agnostic positions, not just on traditional philosophical issues such as the nature of physical causation, but also on current social arrangements, such as state education and the monopolistic power of the medical profession, of both of which she is a relentless critic. Green writes in Letters from Exile and elsewhere of the damage which she believes can be done to exceptional children by holding back, rather than pushing, a topic which she regards as subject to extreme misrepresentation among current educational theorists.
There are also strong hereditarian and anti-feminist elements in her thinking. The former element may have been part of the reason she received support from the psychologist, the late Professor Hans Eysenck, who for a number of years was Director of the Institute of Psychophysical Research which Green founded.
Reinforcing the impression of someone out of sympathy with the modern Zeitgeist is Green's interest in the concepts of royalty and aristocracy. This interest appears to relate, not to their political significance, but to their symbolic power as representing certain ideals of responsibility and self-reliance. In several of her books Green develops a concept of ‘centralisation’, which is far removed from the ‘Californian’ concept of ‘centredness’, and has more to do with a heroic reaction to the perception that the human condition is intolerable, and that single-mindedness and urgency are the only appropriate responses.
To the extent that a conventional political position can be inferred from Green's writings it would appear to be one of extreme libertarianism, and in fact a pamphlet of Green's on education was published in the 1990s by the Libertarian Alliance.
Green's most widely read philosophical book is probably The Human Evasion, which has been translated into Dutch, German, Italian, and Russian. Its tone is somewhat different from Green's other books, being a curious combination of the oracular and the humorous. It consists almost entirely of a destructive analysis of twentieth century thinkers, from Wittgenstein to Tillich, but at the same time it seems to have a positive sub-text of its own, which is never made explicit.
On questions of ethics, Green proposes a distinction between tribal and territorial morality. The latter is largely negative and proscriptive: it defines a person's territory, which is not to be invaded, stolen or damaged, such as his or her property, dependants and family. Outside this defined area territorial morality is permissive, leaving the individual free to have whatever wealth, opinions or behavioural habits that do not harm others.
Tribal morality, by contrast, Green characterises as prescriptive, imposing the norms of a group on the individual. Whereas territorial morality attempts to set up rigid, universal, abstract principles (such as Kant's categorical imperative), tribal morality is contingent, culturally determined, and 'flexible'.
Green links the rise of territorial morality to the development of the concept of private property, and eventually of market capitalism, including the primacy of contract over status. Her evident preference for territorial morality can be related to the centrality of the existential uncertainty in her thinking: under territorial morality it is prohibited to do good to someone against their will because it is impossible for another individual to know with certainty what is in that individual's best interests.
Green's empirical work, some of it undertaken in collaboration with an Oxford psychologist, Charles McCreery, has focussed mainly on hallucinatory experiences in ostensibly normal people.
In 1968 Green published Lucid Dreams, a study of dreams in which the subject is aware that he or she is asleep and dreaming. The possibility of conscious insight during dreams had previously been treated with scepticism by some philosophers and psychologists. However, Green collated both previously published first-hand accounts and the results of longitudinal studies of four subjects of her own. She predicted that lucid dreams would be found to be correlated with the rapid eye movement (REM) stage of sleep, a prediction which was subsequently confirmed by experiment.
Green also speculated that it might be possible to set up a rudimentary two-way signalling system between the lucid dreamer and a waking observer, a possibility which was subsequently realised, independently of each other, by researchers in two different laboratories.
In 1968 Green published an analysis of 400 first-hand accounts of out-of-body experiences. In 1975 Green and McCreery published a similar taxonomy of 'apparitions', or hallucinations in which the viewpoint of the subject was not ostensibly displaced, based on a collection of 1500 first-hand accounts.
Green has put forward the idea that lucid dreams, out-of-body experiences and apparitional experiences have something in common, namely that in all three types of case the subject's field of perception is entirely replaced by a hallucinatory one. In the first two types of case she considers this self-evident from the nature of the experience, but in the case of apparitional experiences in the waking state the idea is far from obvious. The hypothesis, and the evidence and arguments for it, were first put forward in her book Apparitions, and later developed in her book Lucid Dreaming, the Paradox of Consciousness during Sleep, both of which she co-authored with McCreery.
This preoccupation with the extent of the hallucinatory element in various anomalous perceptual experiences is an indication that for Green the main interest of all these experiences is in the light they shed on normal perception, and on our theories of such perception, both philosophical and psychological. Prior to Green's work these various hallucinatory phenomena had been of interest only to parapsychologists, who had studied them with a view to seeing, either whether they provided evidence for extra-sensory perception, or whether they shed light on the question of whether human beings could be said to survive death.
One of Green's most distinctive contributions is to the form of the aphorism or epigram, the majority of her aphorisms being grouped together in two of her books, The Decline and Fall of Science  and Advice to Clever Children. Ten are included in the Penguin Dictionary of Epigrams, a relatively high number for a living author, and three in the Penguin Dictionary of Modern Quotations. The aphorism, with its tendency to paradox and extreme compression, seems to be particularly suited to Green's confrontational mode of thought. For example:
‘In an autocracy, one person has his way; in an aristocracy, a few people have their way; in a democracy, no one has his way.’
‘People have been marrying and bringing up children for centuries now. Nothing has ever come of it.’
‘The way to do research is to attack the facts at the point of greatest astonishment.’ 
with Charles McCreery:
In 1995 Celia Green was involved in the release of a CD entitled Lucid Dreams 0096, narrated by Green for the label Em:t. Earlier Green had contributed a nine-minute track to a compilation CD put out by the same recording label. The track was entitled ‘In the Extreme’ and consisted of readings by the author from her books, The Human Evasion, and Advice to Clever Children.
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