One main function of tarka is said to be the elimination of doubt. Suppose one has the doubt “Is x F?”. One might then judge “if x is not F then x is not G”, and if one also knows that x is G, it will follow that x is F. The Naiyāyikas recognise this function but deny nevertheless that tarka is a source of knowledge (pramāṇa). Their reason, it seems, is that there is a distinction between citing positive empirical evidence for a thesis and merely eliminating the contrary possibility by a priori