This chapter discusses few remarks concerning the role of 'proof by contradiction' in Indian mathematics. Indian mathematical texts do employ the method of indirect proof for proving the non-existence of an entity. There are no instances where Indian mathematicians use the method of indirect proof to establish the existence of an entity, the existence of which is not demonstrable by other means of verification. In this sense, the Indian mathematical tradition may be seen as adopting what is nowadays referred to as the 'constructivist' approach to the issue of mathematical existence. It is important to note that this significant feature of Indian mathematical tradition is closely related to the world view of the Naiyayikas or Indian logicians, who do not accord tarka the status of an independent source of valid knowledge. The tradition of astronomy in India goes back to the ancient texts of Vedangajyotisha which give simple algorithms for fixing the elements of Indian calendar.