In Therese Raquin, I wanted to study temperaments and not characters. This is the entire book. I chose some personages supremely dominated by their nerves and their blood, lacking free will, carried away in each act of their lives by the destinies of their flesh. Therese and Laurent are crude people, in the extreme. I have sought to follow step by step in this brutal work... the pressures of instinct, the cerebral derangement that occurs as the consequence of a nervous crisis. The loves of my two heroes are the satisfaction of a need; the murder which they commit is a consequence of their adultery, [a] consequence which they accept as the wolves accept the slaughter of sheep; finally, that which I have been obliged to call remorse consists of a simple organic disorder, of a rebellion of the nervous system stretched to the breaking point. The soul is perfectly absent, I easily admit, since I had wanted it this way.
It is interesting that Zola does not claim here that there is no such thing as a soul. Rather, he says he wanted to tell a story in which the soul does not exist. The endeavor of Naturalism rests upon certain assumptions about science. Whether those assumptions were right or wrong, a Naturalist writer was bound to follow them through to their logical conclusions. Zola's preface continues:
When my two personages, Therese and Laurent, had been created, I was determined to pose and to resolve certain problems; in this way, I was tempted to explain the strange union that could grow between two different temperaments, I had watched the profound troubles of a fiery nature in contact with a nervous nature.
This vision of the artist as a detached observer, simply performing an experiment and watching to see what happens, became a hallmark of Naturalism. It's important to note that Zola did not necessarily feel this was always necessary. However, to him, it was necessary for the moment. The world had become scientific, and it only seemed (pardon the pun) natural to him that literature should become scientific, too.
The critic Denis Bablet noted that Zola saw Naturalism as the future, as part of a long evolution that might never reach an end. In a discussion of Naturalist dramaturgy, Bablet wrote:
To those who claim to define the essence of theatre, and who believe in the continuity of its conventions, Zola responds that each epoch, each society, each generation has its theatre, which must correspond to its own needs. All his affirmations rest on an evolutionary conception of literature and of art. There is no eternal literature; there is no eternal theatre.
Modern theatre has largely abandoned Naturalism, but perhaps that would not have surprised Zola at all. In fact, the very fact that we have abandoned it might be an indication that Zola was right all along.