  Cognitions that are not memories are classified under two headings: true and false. If we can find factors invariably preceding true cognitions, then we will have the beginnings of a causal theory of knowledge. We will be able to say that a cognition is a knowledge-episode iff it is (i) true and (ii) preceded by a causal factor which invariably precedes true cognitions (i.e. its truth will not be an accident). The Nyāya (TS 39) claim to find four, and only four, invariable correlations: perception, inference, comparison, and testimony. To each they apply the model of causation outlined above. In particular, they isolate, in each case, a ‘special’ instrumental cause (karaṇa), to which they give the name ‘pramāṇa’. In the case of perception, for example, it is the senses. Note that an invariable correlation between two entities, in the sense defined above, does not entail that whenever the first is present, so is the second—the potter is invariably connected with a pot, but only causes a pot when he is actually ‘operating’. Similarly, a pramāṇa, e.g. the eyes, only causes a true cognition when it is actually operating. In other words, the pramāṇa is a necessary but not a sufficient cause for true cognitions. To get a sufficient cause, the Nyāya (TSD 69 iii) says: