In a key passage in Helen McCloy’s 1949 suspense novel Through a Glass, Darkly, a psychiatrist-detective is speaking with a young lady who is at the centre of a storm. Faustina Crayle has been dismissed from her teaching position in a girls’ school under a veil of secrecy, and Dr Basil Willing has discovered the reason: many terrified people believe that Faustina has unearthly powers. Specifically, that she has a silent, ghostly double – or a doppelganger, to use the old German word. Whether Faustina herself is complicit in these spectral sightings – whether she is innocent or malicious – is beside the point for the school’s management. She can’t be allowed to stay on.

Now Basil and Faustina are talking, the former probing gently, the latter trying to make sense of all the disquiet she has caused. And she says:

“Have you any idea what all this is like for me? How desperately I keep asking myself all the old unanswered questions. What is life for? Why were human beings made? Why do we assume so confidently that God is good, when He is so much more likely to be evil? Are we an accident of chemistry, without beginning or end or purpose? Super-colloids, acting out a heartless comedy? Are we a dream of God’s, as the Buddhists believe? Is that why, in early childhood, you stare at your face in the mirror and look at your hands and feet and say to yourself: I am me. I am Faustina Crayle. I am not anyone else. Yet, no matter how hard you try to realise your identity, something inside you goes on feeling that it’s not quite true…”

Out of context, this might sound like a bit of faux-philosophy inserted into a slim, fast-paced mystery. But when you read Through a Glass, Darkly from the beginning, slowly coming to inhabit the world of its characters, the conversation between Faustina and Basil is compelling, poignant, and feels central to the narrative (and not just because it occurs exactly at the book’s midpoint). Cards are placed on the table, a measure of clarity brought to what has so far been a mystifying story. (It is unsurprising that this happens when Basil Willing begins his investigation: he is Helen McCloy’s series detective.) A woman who is an object of suspicion for many of the characters is revealed as a sympathetic, sensitive and weary presence (though of course, we can’t yet be sure that Faustina isn’t other things as well). And then, after the man of science has reassured Faustina that she isn’t in mortal danger from a ghostly double, he walks out into the street, shivers, looks up at the night sky and says to himself: “Who am I to say what cannot happen in this unknowable world?”

At the very end of the book, someone else (I won’t reveal who) will make a similar gesture and pronouncement, and Willing himself will again be faced with self-doubt – even as he plays his role of the detective glibly winding things up.

How much do we know?

Some months ago, when I began rekindling an old passion for crime writing, I found I hadn’t read several authors or books that were celebrated, even regarded as canonical, by serious genre buffs. It’s the sort of thing that can happen when, based on your familiarity with, say, Agatha Christie, Patricia Highsmith, Cornell Woolrich, Raymond Chandler and a few others –authors who tend to be well known to readers splashing about on the shores of the genre rather than plunging fully into it – you think you know almost everything there is to know about Anglophone crime writing of a certain vintage.

High on this list of neglected classics was McCloy’s affecting novel – a book I had never even heard of until I saw it had been ranked at number 12 on a famous 1981 poll of best impossible-crime novels. The four words that make up its title were for me mainly associated with Ingmar Bergman’s great film, which is concerned with the theological implications of the Biblical phrase: the idea that human knowledge is necessarily imperfect, that we will acquire complete vision – with the grace of a higher power – only in the fullness of time.

In a subtly different way, McCloy’s novel also touches on the theme of how limited human knowledge is – how little we yet know about the workings of the human mind, and about the possible links between the corporeal and the non-material worlds.

Consider how a school principal named Mrs Lightfoot tries hesitantly to make sense of the doppelganger phenomenon: “Suppose that an unconscious mind could gather unto itself enough vital energy to project some purely visual image or reflection of itself on the air? Perhaps through some form of refracted radiation? A dream-form that was visible to others as well as to the dreamer – visible but not material [like] reflections in a mirror […] rainbows and mirages.”

Here, a self-described modern woman is struggling to transcend the usual paranoid-sounding language about ghosts or witchcraft and to instead explore the possibility that whatever is going on has an explanation that might make more sense to us once our scientific knowledge has progressed to a certain point.

But it’s all rational, isn’t it – or is it?

In my previous piece in this series, I mentioned books that seemed to be imbued with supernatural occurrences but ended with a logical explanation. Through a Glass, Darkly broadly appears to fit in that category, but to me it felt more like the converse (or a mirror image?). In this case, most of the main characters are devoutly rational, well-educated people whose gut instinct is to be cautious and “sensible”, and the book ends with Basil Willing putting forth his theory, a plausible one. Yet there are still unanswered questions, and little details that one can’t shrug off. For instance, if one attributes the goings-on to a cruel human agency, how does one explain that some of the appearances and actions of Faustina’s “double” play out like her own unrealised impulses?

Perhaps this is McCloy’s major achievement: there is something so viscerally creepy about both her premise and her presentation of it (notably in one scene where two schoolgirls watch Faustina painting in a lawn, her hand moving languidly as if in a slow-motion film; and in another, later scene set in a seaside cottage) that even the most secure reader might not be able to help looking over his shoulder. As with Henry James’s The Turn of the Screw (and its superb 1961 film adaptation The Innocents), about a governess wondering if her young charges are possessed, a sense of claustrophobic unease is created, ensuring that the rational mind is never allowed full control.

The result is that however persuasive Willing’s explanation is, we can’t place ourselves completely in his hands. The lingering impression at the end is not just of a supposed criminal protesting innocence, but also of Faustina herself – a character we have come to feel for – frightened not by an earthly antagonist but by the terror of coming face to face with her own immaterial other-self.

No wonder this is a book that will make you steer clear of reflective surfaces when you’re alone late at night.