Nyayasutras defines inference as the knowledge that follows or derives from other knowledge. It always follows perception, states the text, and is a universal relation or essential principle. One form of inference is a "Purvavat", or as Fowler translates, "from cause to effect or a priori". Thus, if a path or road is wet or river is swollen, states the text, then "it has rained" is a valid knowledge. The sutras assert that the "universal relationship" between the two is necessary for correct, reliable knowledge, that is "if in all cases of A, B is true, then one may correctly infer B whenever A is perceived". Further, there is a causal relation between the two, whether one knows or not of that cause, but inferred knowledge does not require one to know the cause for it to be valid knowledge, states Nyayasutra. The text states one must not confuse coexistence as a universal relation, and that while deduction and induction both are useful and valid means for gaining true knowledge, it lists rules when this method can lead to false knowledge.