Five Characteristics of a hetu:
 It must be present in the paksha, [pakshadharmata] e.g. smoke must be present in the hill.
 It must be present in all positive instances. [sapaksasattva].
 It must be absent in all the negative instances. [vipaksasattva].
 It must be non-incompatible with the minor term [packsha]. [abadita].
 It must be qualified by the absence of counteracting resons which lead to a contradictory conclusions; 'the fact of being caused' should not be used to prove the 'eternality' of sound [Aviruddha].
Gautama classifies inference into:
[a] Purvavat: When we infer the unperceived effect from the perceived cause, then we call it purvavat.
[b] Sheshavat: When we infer the unperceived cause from the perceived effect, we have Sheshavat inference.
[c] Samanyatodrsta: When inference is not based on causation, but on uniformity of co-existence, it is called Samanyatodrasta. We infer creatures with kidneys from the known cases of creatures with heart.
Another classification is:
[a] Kevalanvayi: Mill's Method of agreement. If hetu and sadya are positively related, then it is Kevalanvayi. E.g. All knowable objects are nameable.
[b] Kevalavyatireki: Mill's Method of Difference. If the middle term is the differentium of the minor term and is always negatively related, i.e. terms agree only in absence.