Though Yoga literally means "yoke", "unify", it is also a developed system of philosophy, as well as a practical method to realise the ultimate truth originated in ancient India and codified mainly by Patanjali. Let us look at what makes Yoga different from other systems of thought, so we can understand its metaphysical, ontological, psychological and practical view of life. Historically this may have developed well before Christ and developed side by side other systems of thought in ancient India, but we can clearly see the correlation of Yoga with the Sankhya system. In spite of the connection with the Sankhya doctrines, Yoga differs from it on matters of psychology, ethics etc. and brought a scientific approach to disciplinary practices with rigour and great clarity.