One fine Belfast day in the utterly crazy year of 1972, young Marius Moonston, The Backslider, decides to take a fiver from his sister's purse. But, more important, he decides to take a stand. He cannot find a foothold in an utterly mad Ulster "evangelical" world, with his mad evangelical family and madder society-at-large smothering his questioning mind. His crime opens up a new way of looking at the world, and of acting in it, so his feet gradually find solidity in another mental milieu that better suits his questioning consciousness. His transformation through an act of "theft", his newfound ability to see what is questionable, is the nature of his backsliding, and it is what constitutes the philosophical nature of my novel.

The philosophical novel is the continuation of philosophical reflection by other means. To do justice to the nature of ontological concepts, Plato required a mythological approach in order to illuminate the distinction between essences and existence, which resisted conceptualisation. To do justice to the totality of human experience, existentialism denied objectifying knowledge. Justice was eminently done in some cases, their place in history of philosophical ideas assured and their literary merit lauded. Others failed to hit their desired target but were nonetheless notable for daring to articulate the philosophical idea in this form, and being popularly successful, if not philosophically original.

The Backslider was born out of lived theological prejudice and unease, then philosophical and personal puzzlement that abstract philosophical reflection alone failed to make intelligible. Young Marius Moonston is suddenly a question unto himself: plunged into religious doubt in a country in an equally sudden state of turmoil. Problematical concepts, "salvation", "sin" and "guilt", "judgment" and "condemnation" are prominent. Marius is edging inescapably toward an ethical and ontological response; to resist a powerful milieu and affirm a new way. Important too is the notion of "virtue", which also resists understanding, but is most certainly a problem for philosophy. The theologico-philosophical framework did not necessitate literary approach, but was, nevertheless, given it.

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Spinoza wanted to purify philosophical language of all literary artifice in an uncontaminated Latin. By substituting dynamic Life for Spinoza's Substance Nietzsche required all that ordinary and literary language offered. Yet both thinkers are on the same page philosophically. Reading Zarathustra seriously for the first time, despite its literary form, its poetic vision, I treated it as pure philosophy. But nothing will quite prepare you for the deceptions you will experience with this language. "In order to understand Nietzsche properly you will need the opposite of what a first reading of his works might misleadingly suggest." (Karl Jaspers). The Nazis read it once or not at all.

"Providence did wonderful things through unworthy instruments." The first novel I read by a philosopher, it was described somewhere as a literary psychology, because of Santayana's attachment to William James's ideas on emotions. For James, the emotions are not the bodily processes but the perception of bodily processes. Eschewing a crude psychologism, Santayana develops this idea of the "observing" self philosophically. This work is also a memoir, and the "spirit" of Santayana hovers over it. It is a reflection upon his own understanding. Interesting is the philosophical distinction of the observing "spirit" of the person and the full person, which involves the self-knowledge in act and understanding. Tragedy awaits the person entirely of the spirit.

3. Intimacy by Jean-Paul Sartre

Ordinary young lives. Lulu, Rirette, Henri and Pierre. What of them? Ordinary language. What of it? Familiar circumstances. No Zoroastrian prophet here. No awareness of a single philosophical idea in these minds. No genteel intellectual discussions. Sartre's phenomenology is at work. The idea of "intimacy" denies everything of the transcendental ego and affirms "intentionality", which a detached ego violates. Intimacy is discovered in reflection and interaction, which necessitates the connection to "others". It involves a sacrifice of freedom through commitment and participation, but it is also where we uncover meaning.

Candide is perversion. Voltaire the perverter. Ill-informed about profound religious ideas, he proceeds with caricature. In the history of ideas, that line of "common sense philosophy" leads straight from Voltaire to the modern preachers and prophets of this perversion. Dawkins is an heir of Voltaire. In the name of enlightenment the Frenchman concocts a literary argument against an enlightened metaphysics. Here literature is cowardly. It is journalistic. The enemy of truth that Plato saw in certain literary forms. Voltaire failed to understand rationalist metaphysics and so gave to the world a distortion. The curse of "common sense".

An entertaining piece, very funny with raving mad characters. Murdoch resisted the suggestion that her novels were works of philosophical fiction. Existentialism, for her, defined the whole genre of the philosophical novel. In general, she thought, philosophy clarifies, whereas literature mystifies. She didn't give herself sufficient credit for producing literary works with a subtle philosophical content. "Philosophy makes no progress," she said. So we find ourselves not only discussing the same profound problems we always have but living the same lives we have always lived. So in literature the writer writes about these "same lives" but great literature reveals meanings. A Severed Head is driven by ethical considerations. And we find this at every turn. The philosopher in Murdoch is so much more forceful than the novelist. Finally, a severed head may or may not refer to a disembodied spirit, the Cartesian seat of the emotions. Just a thought.

One of Blanchot's aims was to break down the boundaries between genres. On the one hand this opens the door to philosophical fiction, but on the other it closes it by denying that it is a distinct class. Fiction is doing more than one thing. It seems that he wrote fiction in full awareness that it was, in part, a philosophical investigation. He described the fiction as ontological where the language of ontology departs from literal expression and resides in an area outside the subject and object divide. Ontology speaks analogously. Thomas the Obscure is not familiar literary ground. How could it be? For, essentially, it is a journey of consciousness, an expression of its several characteristics, the relation between ideas and ideas, ideas and their objects, and the conscious states of self-deception.

Anything familiar – plot, narrative, character – have no place here. Heraclitean paradoxes abound. The boundless Thomas is a man with no history. He wanders in a world without the familiar co-ordinates of ordinary experience. And there is no decisive end to it all. There is a circularity that is also epicyclical. Life is repeated in endless cycles.

7. Thérèse Philosophe by Jean-Baptiste de Boyer, Marquis d'Argens

Dostoyevsky referred to it in The Gambler as a "trashy little book". Trashy not in the sense that it was a pornographic paper, a work of light entertainment involving a sexual romp with Catholic clergy confusing sexual with spiritual ecstasy that should never have seen the light of day. Rather, his description points to deep philosophical objections to the intended message, a statement of enlightenment scientific rationalism, especially where it involved the devaluation of transcendent ethical values within a purely mechanistic world-view. De Sade, not a natural ally to Dostoyevsky, saw the very same thing when he says the book "gave us an idea of what an immoral book can do".

The Stranger combines splendidly with Sartre's The Age of Reason. In the latter, Mathieu, a philosophy teacher, is trying to find ways to rid himself of every form of human commitment, hoping that, by doing so, he finds freedom. In doing so, he risks himself, a freedom without a bond is empty and meaningless. It is total subjectivity. He is his own judge and his own victim. On the other hand we have Meursault, in The Stranger, who has no such will to freedom. He is conditioned in every respect. He could not be his own self in his actions "like the mother is in the child" (Nietzsche). Rather than act, he is acted upon, and his world too is empty and meaningless as, in a sense, there is no self.

In the literary field, Eco was influenced greatly by the work of James Joyce, most notably Finnegans Wake and Ulysses. Eco's novel (like Joyce's) expresses the finite and infinite in themes and meanings, but pinned down by what seems to be a simple and fairly accessible detective story set in a medieval monastery. Beyond that simple form, there are for the reader unlimited mysteries to solve and connections to make, in terms of references and relations inside and outside the text. The medieval Sherlock Holmes, William of Baskerville, in the process of his investigations, opens up a hornets' nest of theological interests bubbling under the ordered medieval surface, which, in turn, leads us into a labyrinth of "senses" for the reader to engage. Boundless communication from boundless diversity. Insofar as the philosopher's task is to make reality intelligible, the philosophical significance here is that reality itself, like a text, has that same openness, "an indefinite reserve of meanings" (Eco, The Open Work).

Many readers misunderstood this work. I am not aware of de Botton's response, but mine is that they were off the mark. The complaints were, in the main, twofold. First, frustration that de Botton tells us nothing of what love is, only what it isn't. Second, on the status of this work as a novel, it failed as de Botton had not yet matured as a storyteller. His clever "commentary" is not sufficient compensation.

Both are unfounded. Like complaining that Plato failed in his dialogues to tell us what courage, love, etc. are. Or, in the second case, that Ulysses or Finnegans Wake are failed novels as neither has a story. That the story is weak would recommend it to me. Even more if there was no story. Weaknesses there are, however. Those are philosophical.

• Seán McGrady was raised in Belfast, immersed in the religious and political ideas that defined the Irish Troubles. A former university lecturer in philosophy, he lives in York, England. The Backslider is his first novel.