I once made an informal investigation into the alternative-life fantasies of famous authors. James Joyce coveted the position of agent for Irish tweed in Trieste. Thomas Mann thought he'd make a good banker. Samuel Beckett, if you can believe it, saw himself as an airline pilot manqué. It interested me that even very great writers occasionally yearned to be something other than what they were, but it didn't occur to me how complex the psychology surrounding such yearnings might be.

That psychology is the subject of Adam Phillips's new book, Missing Out. "It is among the contentions of this book," Phillips states in his introduction, "that our unlived lives – the lives we live in fantasy, the wished-for lives – are often more important to us than our so-called lived lives."

It's a fascinating subject, and Missing Out is in many ways a fascinating book, though perhaps not quite the book its introduction leads you to expect. Rather than taking the fantasy itself as his point of departure, and pursuing its implications in the real world, Phillips turns out to be more interested in its originating mechanisms, its inner archaeology, so to speak, which he breaks down into carefully demarcated phases, each with its own chapter: "On Frustration", "On Not Getting It", "On Getting Away With It", "On Getting Out of It" and "On Satisfaction". To the extent that he looks at specific cases, he tends to draw from literature (mostly Shakespeare) rather than life.

Each chapter holds up its title phrase or word for close inspection in the light of etymologies, concordances, psychoanalytic theories and literary texts. "Frustration", for instance, is traced from Robert Cawdrey's 1604 dictionary, through Gloucester's use of "frustrate" in King Lear, to Lear's own catastrophic frustration at Cordelia's refusal to play along with his foolish love-test, and on through the analyst Wilfred Bion's belief in the importance of developing a "capacity for tolerating frustration"; each step layering its own ethical or psychological colouration over the basic idea of not getting what one wants. The word "getting" comes in for even closer attention, its versatility functioning as a kind of reverse prism through which ideas of obtaining, understanding, going, begetting, and so on, are brought together. The underlying methodology, with its meticulous teasing out of suppressed or covert nuances of meaning, comes more or less from Freud, whose legacy seems, increasingly, to be settling in this midway zone between psychology and literary criticism.

It's a fertile but inherently slippery zone to work in, especially if you're trying to construct a densely argued thesis, as Phillips is. With psychology not being quite a hard science, and literature – even King Lear – not being quite life, a faint air of indeterminacy hovers over the book. The mortar between propositions often seems a little soft: you find yourself not so much following the argument as simply reacting to individual statements: great, yes, hm, maybe, I don't think so … It's certainly "worth noticing" (to use a favourite construction of Phillips's) the different ways, for example, in which the phrase "getting out of it" tilts our reading of the narrator's plea in The End of the Affair – "get me out of it" – or Larkin's "Get out as early as you can", or Grace Paley's "Men are always trying to get away in one piece". But there's a limit to what you can build on a foundation of poetry and fiction (they themselves are built out of psychological observation, after all, so that psychologising about them very easily becomes a mirror-game). And interesting as the connections are, they don't always support Phillips's larger assertions, some of which ring a little hollow to my ear, such as: "there is no more fundamental picture of the human subject than as a creature trying to get out of something" (you could just as easily argue the precise opposite).

The places where Phillips permits himself to write from direct professional experience (he's a practising psychoanalyst as well as literary critic) are incomparably more persuasive and engaging, and I wished there were more of them. I'd willingly forgo the etymologies for this kind of human detail: "I remember a child telling me in a session … that the reason he wanted to be bigger was because he wouldn't have to want to be bigger." And I'd gladly trade some of the loftier pronouncements for the more earned generalities of these passages: "What psychoanalysts mostly know about sex is the strange ineffectuality of so much of their knowledge."

Phillips's interest in the unlived life seems to lie mainly in its equivocal nature, as a source of both good and harm; sanity and neurosis. He has very illuminating things to say about the difference between authentic and inauthentic satisfaction, for example, or the way in which the sensation of "getting away with it" links what we admire about rule-breaking artists with what we despise about cynical politicians. He doesn't take a prescriptive or polemical position, but the thrust of his musings is towards the vision of a kind of ideal tension between wanting and getting; fantasising and actualising.

The chapter "On Not Getting It" begins to articulate this vision in literary terms, proposing the state of not "getting" a difficult poem by someone like JH Prynne as in fact a highly desirable state to be in. It's an appealing idea – I'd definitely be up for having my incomprehension of Prynne handed back to me as a form of enlightenment – but at this critical juncture, just where the literary side of the enterprise becomes indispensable to the psychoanalytic side, and where some immersion in a text would have really helped, the writing lapses into wispy rhetoric : "What do you say about a poem – or about and to a person – if the project is not to understand, if the intention is not to get it? If you don't want intelligibility, what do you want instead? Do you stop using the word 'understanding' and start using the word 'redescription'? Do you give an account that does not aspire to be an explanation?"

To some extent this is characteristic of the whole book. It presents itself as a step-by-step analysis of a universal human tendency, but it's really best read as a montage of perceptions and quotations that, like the condition of not-quite-fulfilment it suggests we aspire to, resists clinching its own arguments. Which makes it a more oblique and fragmentary piece of work than it first appears, but perhaps appropriately so.

• James Lasdun's It's Beginning to Hurt is published by Vintage.