The Nyaya logicians classify the inferences in many ways.
1. Basing on the factor, for whom the inference is meant for, inferences are classified into two types, namely, svarthanumana and parartha anumara.
Svarthanumana is meant for the person himself. If I make some inference for me, then there is no need of a detailed logical procedure. Three steps are sufficient to arrive at the conclusion. I can say, for example –
1. The hill has smoke.
2. All cases of smoke are cases of fire.
3. The hill has fire.
This procedure of inference is a psychological one.
Parartha anumana is the inference made for others. So it has to be placed in its perfect logical order. It is logical but not psychological. The five stepped argument discussed earlier is the example of a paratha anumana. It is panca-avayavayavi.
2. From the standpoint of nature of vyapti, inferences are classified into three types, purvavat, sesavat, samanyato drsta
When the unperceived effect is inferred from the perceived cause it is a case of purvavat inference. When we infer good harvest from the timely monsoon, it falls in purvvat category.
When the unperceived cause is inferred from the perceived effect it is a Case of sesavat inference. The inference of rain in the last night by looking to the muddy road in the early morning we have sesavat inference.
This inference is not based on the knowledge of causal relation rather on the relation of co-existence. Basing of the relation of co-existence the presence of one can be inferred from the presence of the other/. Form the fragrance of jasmine flower one can infer the presence of the jasmine flower is a case of Samanyatodrsta inference.
From the standpoint of the method of ascertaining vyapti, inferences are classified in to three types. They are
Kevalanvayi (Agrement in presence)
In this type of inference the universal relation vyapti is established between the middle term and the major term on the basis of a number of positive instances. The conclusion is arrived at on the basis of the presence (anvaya) in agreement. This resembles with Mill’ method of agreement. Eg. All knowable objects are nameable.
The table is a knowable object.
The table is a nameable object.
Kevala vyatireki (agreement in absence)
In this type of inference the universal relation vyapti is established between the middle term and the major term on the basis of agreement in absence i.e. the middle term is only negatively related to the major term. This resembles with Mill’s method of difference. For example:-
No non-fiery object is smoky.
The hill is smoky
the hill is fiery.