17. Spiritism teaches us the mode of union between Spirit and matter through incarnation.
The Spirit, due to its spiritual essence is an unlimited abstract being, which can have no direct action upon matter. An intermediary is necessary to it. This intermediary is the fluidic envelope which makes in some sort an integral part of the Spirit, a semi-material envelope, connecting matter with spirit by its ethereal nature. Like all matter, it is drawn from the universal cosmic fluid, which is submitted by this circumstance to a special modification. This envelope, designated perispirit, from an abstract being, makes of the spirit a concrete, defined being, seized by thought. It renders it apt to act over tangible matter the same as all imponderable fluids, which every one knows are the most potent forces.
The perispiritual fluid is, then, the bond between spirit and matter. During its union with the body it is the vehicle for transmitting thought to different parts of the organism, which acts under the impulsion of will; it also transmits to the spirit the sensation caused by external agents. Its conducting wires are the nerves which are used, as in telegraphing, when the electric fluid has metallic wire for conductor.