Nature is much greater than what a person can perceive through the ordinary senses of the physical body. The hidden aspects of nature consist of finer matter and forces. There is no unbridgeable gulf separating the finer aspects of nature from its gross aspect. They all interpenetrate one another and exist together. The finer aspects of nature are not perceptible to the ordinary individual, but they are nevertheless continuous with the gross aspect that is perceptible to him. They are not remote; and yet they are inaccessible to the consciousness that is functioning through the physical senses, which are not adapted for perceiving those finer aspects of nature. The ordinary person is unconscious of the inner planes, just as someone deaf is unconscious of sounds, and he cannot deal with them consciously. For all practical purposes, therefore, they are other “worlds” for him.
The finer and hidden part of nature has two important divisions, namely, the subtle and the mental, corresponding to the subtle and mental bodies of man. The whole of nature may therefore be conveniently divided into three parts-the gross world, the subtle world, and the mental world. When the individualized soul has incarnated in a physical body, it expresses its life in the gross world. When it drops the outer sheath, the physical body, it continues to have its expression of life either in the subtle world through the subtle body or in the mental world through the mental body.
-Discourses 7th Ed., 304