In Indian logic inference is known as "Anumana". Inference refers to steps in reasoning, moving from premises to logical consequences; etymologically, the word infer means to "carry forward".
The inference is divided into three types: Kevalanvayi, Kevalavyatireki, Anvaya Vyatireki. It is based on the nature of vyapati (the state of pervasion).
Kevalanvayi inference:
- When vyapati between the middle and major term is derived from a uniform agreement in presence alone, it is called Kevalanvayi inference.
- In this inference, the hetu has affirmative concomitance with sadhya only.
- For example: All knowable objects are nameable;
The pot is a knowable object;
Therefore, the pot is nameable.
Therefore, the given example is Kevalanvayi inference.
Kevalavyatireki inference:
- When vyapati between the middle and major term is derived from a uniform agreement in absence alone, it is called as Kevalavyatireki inference.
- In this inference, the hetu is only negatively related to sadhya.
- For example: What is not different from other elements has no smell;
The earth has smell;
Therefore the earth is different from other elements.
Anvaya Vyatireki inference:
- When the middle term in inference is both positively and negatively related to the major term, it is called Anvaya Vyatireki inference.
- In it, there is a universal relation between the hetu and sadhya in respect of both their presence and absence.
- For example: a) All smoky objects are fiery;
The hill is smoky;
Therefore the hill is fiery.
b) No non-fiery object is smoky;
The hill is smoky
Therefore the hill is not non-fiery;
That is, the hill is fiery.
Shesavat inference:
- Here we infer the unperceived cause from a perceived effect.
- For example, when we see a river in flood and infer that there was heavy rain, it is a case of Shesavat inference.