I want to test the ability of students to read a passage and answer questions about it. Could you please come up with a good question for the passage "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)."? 

The forms of thought of this thing that is difficult to determine are?