In plain words
This subtle body too is indeed physical, not different from the gross. It is the abode of subtle powers. That is, the subtle body is where the subtle powers dwell; just as the gross body is a refuge for gross powers, so too the subtle body is a refuge for subtle powers — for the various forms of what is called mental activity. To begin with, there is a body. In it exist gross physical substances and gross power. Power cannot exist without the support of physical substance; for its existence, some physical substance must surely exist. Those very powers become subtler: whichever power functions in gross form — that very power, having taken on a subtle form of activity, becomes thought.
Between these two there is truly no difference at all. Of one and the same thing, one is the gross form and the other the subtle form. In the same way, there is no difference between gross and subtle bodies either. The subtle body too is indeed physical, but the substance within it is subtler. If one seeks the root of all these powers, two substances exist in Nature. One of these is called ākāśa, and the second, prāṇa — these are their names. This alone is power. All that you see, all that you hear, this earth and everything like it — all of it is physical, and all of it is born of ākāśa alone. By the influence of prāṇa's union with it, ākāśa's substance gradually becomes now subtle, now gross. Like ākāśa, prāṇa too is all-pervading and indwells all things. Ākāśa may be compared to a mass of water; and whatever else exists in the world may be compared to the blocks of ice that are born from that water and float upon that same water.
The power that fashions this ākāśa into its various forms is precisely prāṇa. In gross forms prāṇa becomes manifest as sitting, speaking and so on. The gross body is merely an instrument; it is fashioned out of ākāśa's substance, and the subtle body too is fashioned out of ākāśa's substance. But it