  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 methods. This is reflected, perhaps, in the fact that no example is cited in arguments based on tarka to buttress the inference. The Manual of Reason, it seems, goes even further and denies that the counterfactual conditional is a true cognition, which implies, since they are not false either, that counterfactual conditionals do not have a truth-value.