What is the soul in 40k?

[FanSyst]

Reading Liber Chaotica I found that souls are described as being similar to chaos gods, but infinitely weaker. Is there a more reliable definition in 40k?

What is the soul? That is perhaps the most difficult question of all. Just as with an attempt to define the Aethyr in simple terms, it is difficult - nigh on impossible - to know or express what the soul of mortal beings might be is even harder to explain, but answering the question may help us understand the nature of the Chaos Gods. As a priest I was taught (and preached) that the soul is a fragment of the divine within every mortal being, and although this is a simplistic explanation, it served surprisingly well because of it.

Many chaoticians believe that all mortals exist in both the mortal world and the heart of the Aethyr simultaneously. That is to say the body and mind of every mortal exists in the Materium, while an immortal (though unconscious) essence exists within the Immaterium. But do not be deceived, for despite the fact that this essence is supposed to exist within the Immaterium, it is also, and at the same time, within us here upon the Mortal Plane. This does not mean that this essence is split in two, nor does it mean that every mortal has two essences - it means merely that because the Immaterium has no dimension or temporal positioning of its own, it exists everywhere as much as it exists anywhere. As such, one could truly say that this essence is within the Aethyr and separate from the body, and at the same time one could say that the Aethyr is within us along with our essence - both comments are as true, or false, as each other.