Each of these Ages has its appropriate Shastra or Scripture, designed to meet the characteristics and needs of the men who live in them The Hindu Shastra are classed into: (1) Shruti, which commonly includes the four Veda. (Rik, Yajuh, Sama, Atharva, and the Upanishads), the doctrine of which is philosophically exposed in the Vedanta-Darshana. (2) Smriti, such as the Dharma-Shastra of Manu and other works on family and social duty prescribing for pavritti-dhamia, as the Upanishads had revealed the nivritti-dharma. (3) The Puranas, of which, according to the Brahma-vaivartta Purana, there were originally four lakhs, and of which eighteen are now regarded as the principal. (4) The Tantra.

For each of these ages a suitable Shastra is given. The Veda is the root of all Shastra (mula-shastra). All others are based on it. The Tantra is spoken of as a fifth Veda. Kulluka-Bhatta, the celebrated Commentator on Manu, says that Shruti is of two kinds, Vaidik and Tantrik (vaidiki-tantriiki chaiva dvi-vidha shrutih-kirttita). The various Shastras, however, are different presentments of shruti appropriate to the humanity of the age for which they are given. Thus the Tantra is that presentment of shruti which is modelled as regards its ritual to meet the characteristics and infirmities of the Kali-yuga. As men have no longer the capacity, longevity, and moral strength necessary for the application of the Vaidika Karma-kanda, the Tantra prescribes a special sadhana or means or practice of its own, for the attainment of that which is the ultimate and common end of all Shastra. The Kularnava Tantra says that in the Satya or Krita age the Shastra is Shruti (in the sense of the Veda and Upanishads); in Treta-yuga, Smriti (in the sense of the Dharma-Shastra and Shruti-jivika, etc.); in Dvapara Yuga the Purana; and in the last or Kali-yuga the Tantra, which should now be followed by all orthodox Hindu worshippers. The Maha-nirvana and other Tantras and Tantrik works lay down the same rule. The Tantra is also said to contain the very core of the Veda to which, it is described to bear the relation of the Paramatma to the Jivatma. In a similar way, Kaulachara is the central informing life of the gross body called vedachara, each of the achara which follow it up to kaulachara being more and more subtle sheaths.