The following is an excerpt taken from the Grimoire of Undeath (Mostly used by first time necromancers) explaining the Trinity of Being.
The Trinity of Being is what all living things are made out of. It is composed of the Body, Mind, and Soul, each of which is crucial to the functioning of the being. Of these, strangely enough to anyone non-familiar to the inner workings of the being, the Mind is the weakest of these three components. The Body is hardy, enduring things that kill the Mind. See, if you were stabbed in the heart, you would die. It's a simple thing that anyone with half a brain could tell you. The process of dying, however, is much different than you might think. Upon a fatal attack on the being, most specifically a human being, the Body is injured, and the Mind is shattered. The Soul remains intact, but we'll get back to that later on in this passage.The Mind is lost, and the Body, without any input, falls limp. The Soul is later removed by the Seeker.
The Soul, unlike the Body or the Mind, is entirely indestructible by any means possessed by mankind at this moment. It is the glue that holds the Body and the Mind together in one vessel. Some other necromancers say that with the Soul of a dead creature, you could reconstruct a shattered mind of that same dead creature, though due to the efforts of the Seeker, that hypothesis has yet to be tested.
The Mind of the being is the central command of the entire being, the component that runs the entire vessel. It assumes command of the Body, which it can do because of help from the Soul, and because of this link between the Body and the Mind, an attack that affects the Body also affects the Mind, and since the Mind is much less durable than the Body, it results in the basic stab someone and they die system. The Mind is centralized in the brain, an organ specifically meant to channel the Mind through the link the Soul provides.
The Body is, well, the body. Without the Mind, it's just a sack of flesh, but with it, it's a complex lifeform. Contrary to what most might believe, the Body is extraordinarily resilient, most fatal attacks one might suspect to harm it mostly just targeting the Mind. The way one would destroy the Body is to mutilate, burn, or otherwise destroy it to a point where the original form is no longer recognizable.
End of excerpt.