"Rudra is truly one; for the knowers of Brahman do not admit the existence of a second". The period of 200 BC to 100 AD also marks the beginning of the Shaiva tradition focused on the worship of Shiva as evidenced in other literature of this period. Other scholars such as Robert Hume and Doris Srinivasan state that the Shvetashvatara Upanishad presents pluralism, pantheism, or henotheism, rather than being a text just on Shiva theism. Shaiva devotees and ascetics are mentioned in Patanjali's Mahābhāṣya (2nd-century BCE) and in the Mahabharata. The earliest iconic artworks of Shiva may be from Gandhara and northwest parts of ancient India. There is some uncertainty as the artwork that has survived is damaged and they show some overlap with meditative Buddha-related artwork, but the presence of Shiva's trident and phallic symbolism in this art suggests it was likely Shiva. Numismatics research suggests that numerous coins of the ancient Kushan Empire (30–375 CE) that have survived, were images of a god who is probably Shiva. The Shiva in Kushan coins is referred to as Oesho of unclear etymology and origins, but the simultaneous presence of Indra and Shiva in the Kushan era artwork suggest that they were revered deities by the start of the Kushan Empire. The Shaiva Upanishads are a group of 14 minor Upanishads of Hinduism variously dated from the last centuries of the 1st millennium BCE through the 17th century. These extol Shiva as the metaphysical unchanging reality Brahman and the Atman (Self), and include sections about rites and symbolisms related to Shiva. The Shaiva Puranas, particularly the Shiva Purana and the Linga Purana, present the various aspects of Shiva, mythologies, cosmology and pilgrimage (Tirtha) associated with him. The Shiva-related Tantra literature, composed between the 8th and 11th centuries, are regarded in devotional dualistic Shaivism as Sruti. Dualistic Shaiva Agamas which consider Self within each living being and Shiva as two separate realities (dualism, dvaita), are the foundational texts for Shaiva Siddhanta. Other Shaiva Agamas teach that these are one reality (monism, advaita), and that Shiva is the Self, the perfection and truth within each living being. In Shiva related sub-traditions, there are ten dualistic Agama texts, eighteen qualified monism-cum-dualism Agama texts and sixty-four monism Agama texts. Shiva-related literature developed extensively across India in the 1st millennium CE and through the 13th century, particularly in Kashmir and Tamil Shaiva traditions. Shaivism gained immense