Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Objective, scientific, unbiased, nightmarish
Okay, so you're probably nodding your head as you read the first three adjectives we've used to described the tone of the novel. Objective: check. Scientific: check. Unbiased: check. Zola was trying to present Thérèse Raquin as a scientific study devoid of moral judgments.
So of course it's all objective and scientific and unbiased.
But then you got to that last adjective: nightmarish. And that one kind of throws a wrench in the whole system, you know? According to what he says, Zola did not intend for anything in the book to be remotely supernatural—because the supernatural is the arch-nemesis of the empirical.
And yet, weren't there moments in Thérèse Raquin when you felt as if you were reading a horror story? Zola constantly uses adjectives like "sinister" and "atrocious," or nouns like "crisis," "anguish," "terror," and "fear," right? Like in this passage, which describes Laurent's mental state after the murder of Camille:
As soon as night fell, as soon as he was shut in with his wife, he came out in a cold sweat and childish terrors assailed him. In this way, he went through periodic crises, nervous attacks that returned every evening and deranged his senses by showing him the grotesque green face of his victim. It was like the onset of a terrifying disease, a sort of hysteria of murder: the words 'illness' and 'nervous affliction' were really the only ones that could properly describe Laurent's fears. (22.5)
Alright, so the narrator tries to diagnose Laurent's terror by explaining it in medical terms. It's a "disease," "illness," or "nervous affliction." There's no discussion of Laurent's internal psychology.
But does this passage really sound all that scientific to you?
Despite the emphasis here on Laurent's bodily "illness," the whole ordeal sounds a lot like a horror story. There are so many words and phrases typically associated with the horror genre: "cold sweat," "childish terrors," "crisis," "nervous attacks," "deranged," "grotesque," "terrifying," "hysteria," "fears." Yeah. It's a mixed bag up in here.