Traditionally, the Veda is regarded as the source of all adhyātma-vidyā. However, several śaiva, śākta, and Vaiṣṇava āgamas frequently categorize Vaidika-vidyā as laukika and associate adhyātma vidyā exclusively with Tantra and Yoga-śāstra. A modern commentator on Paraśurāma Kalpasūtra discusses this at length when elaborating on the sūtra that refers to śrīvidyā Tantra as kulastrī while comparing Vēdādi laukika vidyās to prostitutes. The reasons for this exaggeration are numerous and many a time, well-justified.
laukikaṁ vaidikaṁ cāpi tathādhyātmikamēva ca |
trividhaṁ tatsamuddiṣṭaṁ pradhānaṁ cōttarōttaram ||
laukikaṁ tvarthaśāstrādi vaidikaṁ vēdasaṁyutam |
adhyātmajñānamanyacca yōgatantrādi kathyatē ||
A commentator on this āgamic verse includes the Upaniṣads in the group of Yoga and Tantra. Also, note the use of the phrase – pradhānam cōttarōttaram. One should also carefully consider the phenomenon of rebellious Aupaniṣadas of Pre-Buddhistic (and obviously pre-Shankara) period who had serious objections to what was prevalent at that point as śruti (and needless to say also the smr̥ti) and considered the Upaniṣad to be the only true śruti, rejecting the rest as teachings aimed at generating kṣudra phala. It is extremely likely that the śramaṇas and their offspring schools were heavily influenced by this class of rebels, as posited by several scholars.
An ancient commentator on the Pātañjala sūtra states: Yajña is associated with paśu in every way, as the Yajamāna is also a paśu (on account of his bondage to the Yajña-phala as also the which he sees as kṣudra-phala) albeit belonging to comparatively, an uttama jāti. On this basis, he expresses reservation on associating the term Yajña with Yoga. Several traditional ācāryas (including Chitsukhācārya who is explicit in this regard) consider the Upaniṣad as the culmination or the height of Vedic thought and other śāstras including the non-jñāna kāṇḍa of the śruti as mere precursors. Agamas and āgamikas (such as Mahāmāhēśvara Abhinavagupta) attempt to present a similar case in their writings from the perspectives of their own schools of thought with varying levels of assertiveness. That several traditional commentators have had to justify Upaniṣads as valid śruti pramāṇa is another aspect that needs to be considered here.
Analysis of the numerous philosophical schools cannot be complete without a clear understanding of their varied goals. An assumption that every school/practice leads to the same goal misses the crucial concept of ‘Right View’. The modern-day Hindus would certainly benefit from studying some crisp writings of the Bauddha philosopher Nāgārjuna in this regard. Though Nāgārjuna writes nothing new that one cannot find in the gamut of āstika darśanas, his clear presentation certainly takes the cake.