The third portion is the Desire Body, so called because man's feeling and passional nature finds in this its special vehicle. In sleep, the man leaves the physical body, and carries on his conscious activities in this, which functions in the invisible world closest to our visible earth. It is therefore his vehicle of consciousness in the lowest of the super-physical worlds, which is also the first world into which men pass at death.
The fourth portion is the Mental Body, so called because man's intellectual nature, so far as it deals with the concrete, functions in this. It is his vehicle of consciousness in the second of the super-physical worlds, which is also the second, or lower heavenly world, into which men pass after death, when freed from the world alluded to in the preceding paragraph.
These four portions of his encircling form, made up of the dual physical body, the desire body, and the mental body, form the natural body of which S. Paul speaks.
This scientific analysis has fallen out of the ordinary Christian teaching, which is vague and confused on this matter. It is not that the churches have never possessed it; on the contrary, this knowledge of the constitution of man formed part of the teachings in the Lesser Mysteries; the simple division into Spirit, Soul, and Body was exoteric, the first rough and ready division given as a foundation. The subdivision as regards the "Body" was made in the course of later instruction, as a preliminary to the training by which the instructor enabled his pupil to separate one vehicle from another, and to use each as a vehicle of consciousness in its appropriate region.
This conception should be readily enough grasped. If a man wants to travel on the solid earth, he uses as his vehicle a carriage or a train. If he wants to travel on the liquid seas, he changes his vehicle, and takes a ship. If he wants to travel in the air, he changes his vehicle again and uses a balloon. He is the same man throughout, but he is using three different vehicles, according to the kind of matter he wants to travel in. The analogy is rough and inadequate, but it is not misleading. When a man is busy in the physical world, his vehicle is the physical body, and his consciousness works in and through that body. When he passes into the world beyond the physical, in sleep and at death, his vehicle is the desire body, and he may learn to use this consciously, as he uses the physical consciously. He already uses it unconsciously every day of his life when he is feeling and desiring, as well as every night of his life. When he goes on into the heavenly world after death, his vehicle is the mental body, and this also he is daily using, when he is thinking, and there would be no thought in the brain were there none in the mental body.