A material theory of soul or spirit is found in some ancient Eastern teachings. The concept is presented in more modern, Westernized language by G.I. Gurdjieff in his theory of the “Food Factory”. Gurdjieff says that “everything is material” including the soul. A soul is not present ab initio in humans but must be grown from the proper material which is produced by self observation and intentional suffering. Not knowing how to do this most humans, much less the unborn, have no soul.
Curiously this theory is mentioned in Monty Python’s “The Meaning of Life” in the Very Big Corporation of America board meeting scene.
I’m agnostic about Gurdjieff’s theory of a material soul. IMO the soul is best considered as a constellation of cognitive capacities like instinct, intuition, empathy, love, etc. However, those capacities do have a material substrate in the form of the brain, CNS, and other tissues and organs of the human biocomputer, and they are capable of progressive development which is no doubt reflected in corresponding changes in the material substrates. In addition to self observation and intentional suffering, the soulful aspects of the human biocomputer can be further developed through nutrition, cultivating good gut microbes, and many kinds of practice like meditation and selfless good works, mindfully performed. Growing a soul is thus like feeding and exercising a muscle. Or a baby.
But it doesn’t happen by accident and it’s not automatic.