The Bhagavad Gita had been made practically the only source for the means to moksha with the development of Classical Hinduism in the 8th or 9th century, and Hindu philosophers of the medieval period have tried to explain the nature of these three paths and the relation between them.
The 12th-century philosopher Ramanuja considered the three yog by interpreting his predecessor Yamunacharya. In Ramanujam's interpretation, bhakti-yog appears to be the direct path to moksh, which is however available only to those whose inner faculties have already been trained by both karma-yog and gyan-yog.
A "fourth yog" is sometimes added, Raj Yog or "the Path of Meditation". This is the classical Yog presented in the Yog Sutr of Patanjali. Patanjali's system came to be known as Raj Yog or "Royal Yog" retro-actively, in about the 15th century, as the term Yog had become popular for the general concept of a "religious path".
The systematic presentation of Hindu monotheism as divided into these four paths or "Yog" is modern, advocated by Swami Vivekananda from the 1890s. They are presented as four paths to God suitable for four human temperaments, viz. the active, the emotional, the mystic and the philosophical.
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