Through my years of study in Occult Science, one field of interest has always been the soul and its relation/coloration to the life force. It was always understood that the more demanding the magickal feat, the more soul/life force was required to accomplish it. The exchange is always prevalent and there is no magick, big or small without exchange and certainly none without consequence.
So, to give an example, returning a soul to its original earthly vessel would take out of you or a host of your choosing, a considerable amount of life force/ soul than would let's say casting a curse on someone. If you use up too much of your soul you cease to exist. Should this occur your soul disappears into the void, there is no afterlife for something without a soul. It is my understanding that the Universe has a finite amount of souls not infinite, hence the reincarnation process, in this plane of existence or another.
So should there be no host available, then you need to be the primary source. The issue now arises of how to replenish that life force/soul that you have used up to perform whatever task you were presented with. It is known that the longer you take to replenish that life force, the danger of falling seriously ill becomes far more realistic.
I have found that life force can be replenished in various ways from hosts around you, the trick here is not to drain that host to a dangerous level, but to take only what is required. Sexual activity, touch, or indeed blood, are but some ways with which life force can be regenerated.
So what is life force? Witches, Luciferians/Theosophists, New Agers, and others regard life force as "the Universal Energy that gives life to all things. They believe and teach that life force gives life. Christians believe and teach according to Scripture, not occult doctrine. The Word ( Jesus Christ) is the creator. Life is in him, not life force energy.
Life Force Energy is also the power used in magick. Magick is simply the act of directing Life Force Energy through the power of Will to effect change in the manifested Cosmos. Some names for life force energy are; Universal Life Force, vital force, healing force, elemental energy, Aka, Prana, Chi/Qi/Ki, psychic healing, biofield, Kundalini, Spirit Energy, Mana, divine energy, Orgone, animal magnetism, para electricity.
The power of life force energy is demons, although this fact is almost never stated plainly by those who teach about it and use it. The demons are alive (life) and they have the ability to do work by supernatural means (force) but they do not give life.
You are a soul, you have a body. I like to think that a soul inhabits a body and the life force is what keeps the body moving. Soul and life force to some are two separate things. Others believe that Lifeforce is tied to the body and the soul is tied to the spirit. When a vampire drains you of your life force, you die.
The Soul is personalized energy, partially overlaying (but not identical) with your aura, that holds all your memories, emotion, and personality, but not in its whole structure, they are elements that are just made out of energy created by living and experiencing positive emotions and connection to other people. Life Force or Chi/Qi/Mona etc. is the mystical energy that comes with being alive. If your cells have active mitochondria you have life force. Now we have a bit of a chicken&eggs situation here so it's unclear which fuels which but you can't live without both. It is what your aura is made out of, what Vampires would feed on. It's the energy that a Magus would tap into if they can't draw from the surrounding environment.
Your soul permeates everything in your body but is not a part of the body. The soul can exist without the body, but not the other way around. Your soul is the essence of free will. And no one can touch that. No one can take away your free will, and so, nothing can take away your soul. Except you. The one thing you can't protect yourself from is yourself. That being said, I'm sure your soul 'flavors' your want for life. You are your soul, controlling the actions of the body.
To me, the soul and the life force are merely two sides of the same coin. As stated by others, the soul animates the spirit, while the life force animates the body. Neither can exist without the other. No soul, no lifeforce, and vice versa. The soul is the part that is undeniably you. Lifeforce is like the glue holding your soul to your physical body.
Ii-wy em Hotep - Patrick Gaffiero