There is one ancient Indian text on it and it is called Taittiriya Upanishad.
The five layers or panca kosha vishleshana are according to that text inteconected sheaths of the Self. Each sheath is subtler than the other:
The first sheath is the physical sheath which is food, air, water or annamaya kosha.
The second is the sheath of vital energy or pranamaya kosha.
The third is the mental sheath, the conscious experience, or manomaya kosha.
The fourth is the intuitive experience of the outer world or vijnanamaya kosha.
The fifth is anandamaya kosha which is the experience of unification with the cosmic prana, with the higher knowledge where all other forces melt into each other (Yoga generally means unity, harmony).
The first tree sheets are connected to the matter and the last two to the spirit.