The Vedas, Upanishads, Ithihasas and Puranas were authored over a period of around 2000 years with thousands of people being the authors. This is what modern historians have concluded and though one might dispute the dates given by them it is extremely hard to argue even from traditional beliefs that they were authored at a single time or by a single person. On the other hand, most of the great philosophers proceed on the assumption that all these texts espouse one coherent and consistent philosophy. Adi Shankara thus has written many commentaries on the Upanishads and Bhagavad Gita interpreting the texts to fit with Advaita. Similarly for Ramanuja and Madhva. However, what argument can be given to say that all these manifold texts have a single coherent viewpoint? Have the great acharayas given any argument or do they merely take it as an assumption?