Hindu philosophy also includes several sub-schools of theistic philosophies that integrate ideas from two or more of the six orthodox philosophies, such as the realism of the Nyāya, the naturalism of the Vaiśeṣika, the dualism of the Sāṅkhya, the monism and knowledge of Self as essential to liberation of Advaita, the self-discipline of yoga and the asceticism and elements of theistic ideas. How many subschools does Hindu philosophy have?