Hindu esoteric literature spans more than a millennium of largely tantric and yogic writing concerned with hidden knowledge, subtle physiology and techniques of realisation, distinct in both period and purpose from the philosophical argument of the classical Upanishads. The Tantras themselves, composed roughly from the sixth century onward and organized around lineages devoted to Shiva, Vishnu or the Goddess, describe ritual, mantra and the visualisation of deities as means to a liberation available within this very body, in this very life. Later Hatha Yoga manuals, including the Hatha Yoga Pradipika, the Gheranda Samhita and the Shiva Samhita, systematised the subtle-body model these currents share: a network of channels, nadis, and centres, cakras, through which a vital energy is directed by breath control and posture. The Vijnana Bhairava Tantra, a Kashmiri Shaiva text of roughly the eighth century or earlier, sets out well over a hundred short meditative techniques rather than a single doctrine, and remains one of the most widely translated works of this literature outside India.
What unites the texts gathered under this heading is not a single lineage or century but a shared concern: knowledge treated as something to be realised in practice rather than settled by argument, addressed to an initiate rather than to a general reader. Samuel's history of these currents traces how far the subtle-body vocabulary now associated with popular yoga, cakra, nadi, kundalini, actually developed inside this specific tantric and yogic literature rather than in the older Vedic or Upanishadic corpus with which it is often conflated.