The second edition of Thérèse Raquin contains a preface, a defense of the scandalous novel, that I suspect Émile Zola had at the ready. He was a savvy character. "I am charmed to observe that my fellow writers have the sensitive nerves of young girls" - that's the general tone. Here's what he says he meant to do:
I chose to portray individuals existing under the sovereign dominion of their nerves and their blood, devoid of free will and drawn into every act of their lives by the inescapable promptings of their flesh. Thérèse and Laurent are human animals nothing more. In these animals I set out to trace, step by step, the hidden workings of the passions, the urges of instinct, and the derangements of the brain which follow on from a nervous crisis... There is a total absence of soul, as I will readily admit, for such was my intention.
Zola is a clinician, a writer in a lab coat, "a doctor lecturing to students about disease." Is Zola describing his own book adequately? Up to a point, Lord Copper. I've been trying to argue that side myself, for the last day or two. But.
An adulterous couple murder the inconvenient husband and marry. However, they suffer from an "organic disorder" that in another novel would be called guilt, so even though they marry, they cannot have sex. Why not? Because they simultaneously hallucinate the decayed corpse of the dead man, right there in the bedroom, right between them in bed, every night. Both of them see and feel it. I'm not sure how the story, as opposed to the concept, would be different if there were a plain old ghost.
But no, this is science. The shared hallucinations are the result of "a kind of equilibrium between them based on the complementarity of their organisms." The woman's nerves are over-wrought, the man is in a "state of nervous hypersensitivity," like "a young girl suffering from an acute neurotic condition" (he's like one of the novel's critics!). When the narrator claims that "[t]heirs was the inept hypocrisy of two insane people," I was prepared to agree, although I am not sure what "inept hypocrisy" is:
It would be interesting to study the changes, determined by circumstances, which sometimes come about in certain organisms; these changes begin in the body but quickly communicate themselves to the brain, and thence to the whole individual.
All of this has been from Chapter XXII, about two-thirds of the way through the novel. The great climax of the dual hallucination theme comes in the next chapter, when the inept hypocrites simultaneously realize that they might be able to exorcise the ghost through sex:
They hugged each other in a horrible embrace. Pain and terror took the place of desire. When their limbs touched, they felt as if they had fallen on to a pile of glowing embers. They let out a scream and clung more tightly to each other, so as not to leave any room for the [murdered] man between their bodies. But they could still feel pieces of him squashed revoltingly between them, freezing their skin in some places while the rest of them was burning hot.
My guess is that, confronted with this passage, some readers will permanently excise Zola from their lifetime reading plan, while others will add him. That last sentence - "pieces of him squashed" and so on - not every writer will give you that! Thank goodness. And I'm not even going to get into the scar. The horrible pulsating scar!
None of this is science. None of this is based on close observation, or documentation, or any of the other attributes of Naturalism. This is all made up. Fiction. It's intense, repulsive, exciting, vivid, nuts. This is Expressionism, forty years in advance - emotions and images pushed to extremes, realism be damned. Or maybe it's just horror writing. It's effective. But natural, or Natural? Émile, I'm on to you.