The earliest reference to a six-phased yoga is found in the Maitrāyaṇīya, or Maitrī Upaniṣad, which belongs to the branch of Kṛṣṇa Yajurveda and is considered to be the last of the classical Upaniṣads. The ṣaḍaṅga yoga of the Maitrāyaṇīya Upaniṣad, Ch. 6, v.18, contains the following six phases:
(a) Prāṇāyāma (breath-control)
(b) Pratyāhāra (retraction)
(c) Dhyāna (meditative stabilization)
(d) Dhāraṇā (concentration)
(e) Tarka (contemplative inquiry) and
(f) Samādhi
It is taught in this Upaniṣad as a method for achieving union with the Supreme Self (paramātman). If we accept that the Maitrāyaṇīya Upaniṣad predates Patanjali, we can assume that this six-phased yoga also predates the eight-phased yoga (aṣṭāṅga yoha) of the classical Yoga system. The fact that Patanjali never makes any reference to a six-phased yoga and that his Yogasūtra never mentions contemplative inquiry (Tarka) is not sufficient evidence to regard six-phased yoga as a later revision of the eight-phased yoga, as Gunter Gronbold suggests. Even if the sixth chapter of the Maitrāyaṇīya Upaniṣad, which incorporates a six-phased yoga, is a later interpolation, as Mircea Eliade speculates, the antecedence of the six-phased yoga to the yoga of Patanjali is still quite plausible. The phrase ‘for it is said elsewhere’, which often occurs at the beginning of the verses of the sixth chapter, indicates that the Maitrāyaṇīya Upaniṣad draws its yogic elements from the earlier yogic sources. Even though we are unable to determine the exact sources of the yogic elements in the Maitrāyaṇīya Upaniṣad, it is obvious that different forms of its isx-phased yoga have very early origins in India. The six-phased yoga was later modified into diverse forms of yoga with varying numbers of phases.
For example, in one of the earliest Purāṇas, the Vāyu Purāṇa, Ch. 10, v.76, one encounters a five-phased yoga, whose fifth phase is Smaraṇa (recollection), corresponding in name to the fifth phase of the Buddhist Kālacakratantras six-phased yoga. In this Purāṇa as in the Kālacakratantra, contemplative inquiry (Tarka) is replaced by recollection. Considering that the Purāṇas underwent many revisions after the majority of their material was composed during the Gupta reign (c. 320-c.500 CE), it is extremely difficult to establish whether the recollection phase of yoga was established first in the Purāṇic tradition or in the Buddhist tradition, specially, in the Guhyasamājatantra, which some scholars date as early as the fourth century CE and some as late as the eighth century CE.
Within later Hindu sources, a six-phased yoga is also mentioned in a number of texts belonging to the Upaniṣadas of the Yoga class – specifically, in the Amṛtabindu Upaniṣad – and in the Shaiva Agamas, Shaiva tantras and some Dharma Sūtras, where there is a slightly different order of phases than that found in the six-phased yoga in the Maitrāyaṇīya Upaniṣad. For example, in the Amṛtabindu Upaniṣad, v.6, the six phases of yoga are pratyāhāra, dhyāna, prāṇāyāma, dhāraṇā, tarka and samādhi. This particular sequence of the phases of yoga is almost identical to that of the Kālacakratantra. The difference between the two lies in the designation of the fifth phase of yoga as Tarka instead of Anusmṛti. Even though contemplative inquiry (Tarka) is not explicitly mentioned among the six phases of the Kālacakratantra’s six-phased yoga as a separate member, it is not absent from there. Rather, it is included within the phase of Dhyāna, along with Prajñā (wisdom), Vicāra (analysis), Rati (joy) and Achala-sukha (immutable bliss). Contemplative inquiry as a constituent of the phase of meditative stabilization is explained in the Vimalaprabhā as the apprehension of the phenomenon of empty form that is being observed or meditated upon during this phase. As such, it is an indispensable element in the practice of the Kālacakratantra’s six-phased yoga. Nevertheless, it is not given superiority over all other phases of the six-phased yoga and their elements as it is in Kashmir Shaivism – specifically in the Shaivagamas and in the works of Abhinavagupta and Jayaratha. Abhinavagupta asserts that “among all the lights of the component parts of yoga”, contemplative inquiry (tarka) has already been determined in the earlier Mālinīvijaya “to be the brilliant sun by which one gets liberated and liberates others.” When commenting on Abhinavagupta’s Tantrāloka, Jayaratha (thirteenth century) in his Tantrālokaviveka mentions the six-phased yoga that has prāṇāyāma as its first member and Tarka or contemplative inquiry as its fifth member and exalts it as the highest phase. Moreover, just as contemplative inquiry is included in the six-phased yoga of the Kālacakratantra, even though it is not regarded as a separate phase, so too are meditative posture (āsana) and restraint (niyama) implicitly included in this yoga. The vajra posture (vajrāsana) is often referred to as the posture in which an adept of the Kālacakratantra does his meditative practice, whereas niyama is included in the observance of the Kālacakratantra’s ethical discipline, in the form of restraint from indulging in the five objects of desire and keeping the twenty-five tantric percepts, which are deemed prerequisites for the successful outcome of the practice of the six-phased yoga. The Vimalaprabhā defines niyama as a Buddha’s command (buddhānu~jnā) with regard to the twenty-five precepts. Since these two prerequisites to the Kālacakratantra’s six-phased yoga are present in each phase of the yoga as qualifying conditions, they are not considered to be separate phases.
Within later Hindu sources there are also those who speak of a six-phased yoga that does not include the phase of contemplative inquiry but includes meditative posture (āsana) as the first phase. For example, some Yoga Upaniṣadas, specifically, the Dhyānabindu Upaniṣad, v41, and the Yogacūḍāmaṇi UpaniṣaD, v.2 – several texts of the Gorakṣa corpus (c. twelfth century), and the Netratantra, cited in Kṣemarāja’s Vimarśinī (eleventh century) commentary on the Shiva Sūtra 6, contain the following list of six phases: āsana, prāṇāyāma, pratyāhāra, dhyāna, dhāraṇā and samādhi. This form of the six-phased yoga seems to be later than that found in the Guhyasamājatantra and later incorporated into the Kālacakratantra. Thus, it is most likely that the Buddhist six-phased yoga chronologically succeeds the six-phased yogas containing Tarka as the fifth phase, which continued to be in practice in later times as well. However, it is most difficult to determine with certainty whether the Buddhist six-phased yoga precedes the six-phased yoga of Kashmir Shaivism that contains āsana as its first phase or whether it was contemporaneous with it. If one were to rely only on the extant Shaiva texts that refer to the six-phased yoga having meditative posture as its first member, it would seem that the Buddhist six-phased yoga preceded that particular yoga of Kashmir Shaivism. Considering the incompleteness of textual and historical information, it is impossible to reconstruct an accurate and precise history of the six-phased yoga in India. Therefore, I offer here only a limited comparative list of the different types of six-phased yogas that were cited in specific Shaiva, Vaiṣnava and Buddhist texts.
The Tarka class of ṣaḍaṅga-yoga:
Maitrāyaṇīya Upaniṣad
Prāṇāyāma
Pratyāhāra
Dhyāna
Dhāraṇā
Tarka
Samādhi
Yoga Upaniṣads such as Amṛtanāda etc.
Pratyāhāra
Dhyāna
Prāṇāyāma
Dhāraṇā
Tarka
Samādhi
Viṣṇu Samhitā
Prāṇāyāma
Pratyāhāra
Dhāraṇā
Tarka
Samādhi
Dhyāna
Tantrālokaviveka
Prāṇāyāma
Dhyāna
Pratyāhāra
Dhāraṇā
Tarka
Samādhi
The Anusmṛti class of ṣaḍaṅga-yoga:
Guhyasamājatantra and Kālacakratantra
Pratyāhāra
Dhyāna
Prāṇāyāma
Dhāraṇā
Anusmṛti
Samādhi
The āsana class of ṣaḍaṅga-yoga:
Netratantra
Asana
Prāṇāyāma
Pratyāhāra
Dhyāna
Dhāraṇā
Samādhi
Gorakṣa Texts, Gorakṣaśataka etc.
Asana
Prāṇāyāma
Pratyāhāra
Dhāraṇā
Dhyāna
Samādhi
As this list indicates, not only teachers of different religious traditions but also various teachers of different schools within the same tradition taught diverse forms of the six-phased yoga, according to their intended goals. Even though these diverse forms of six-phased yoga were couched within the different theoretical and practical frameworks of disparate traditions, they all share some commonalities. The most salient point of commonality is that each form of six-phased yoga is viewed within its own tradition as inducive to the accomplishment of both limited, or mundane, and supreme siddhis. There are also certain commonalities in the more general interpretations of some phases of the diverse types of six-phased yoga, despite the clear divergence in the manner in which particular phases are structured and practiced within different traditions. For example, in both Kashmir Shaivism and Buddhism, the phase of breath control (prāṇāyāma) involves bringing the prāṇas into the central ṇāḍī; the phase of retraction (pratyāhāra) involves the withdrawal of the senses from external objects; and dhyāna implies meditation on a divine form, and so on. Their interpretations coincide to a certain degree with Patanjali’s definitions in the Yoga Sūtras.