Early history of Shaivism is difficult to determine. However, the Śvetāśvatara Upanishad (400 – 200 BCE) is considered to be the earliest textual exposition of a systematic philosophy of Shaivism. Shaivism is represented by various philosophical schools, including non-dualist (abheda), dualist (bheda), and non-dualist-with-dualist (bhedābheda) perspectives. Vidyaranya in his works mentions three major schools of Shaiva thought— Pashupata Shaivism, Shaiva Siddhanta and Pratyabhijña (Kashmir Shaivism).