The form of the argument has five steps (panca avayabi). The basis of drawing the conclusion is vyapti.
The vyapti is the relation of constant concomitance between hetu and sadhya. The vyapti is the nerve of inference expressed in a universal statement. Naiyayikas provide six grounds for the ascertainment of vyapit.
Later Buddhist logicians attach maximum emphasis on the causal relationship for the ascertainment of vyapti. Vedantins have accepted the relation of vyapti to be contingent instead of being a necessary one. They follow the method of repeated observations for the ascertainment of vyapti.
There are five types of fallacies (Hetvabhasa) they are – Savyabhicara, Virudha, Satapratipaksa, Asidha and Badhita. Inferences are classified in many ways.
According to one classification inferences are of two types
i) Svarthanumana- Inference for the self. (3 steps)
ii) Parartha numana- Inference for others.(Ssteps)
According to another classification Nature of vyapti) inferences are classified in to three types.
i) Purvavat- unperceived effect from perceived cause.
ii) Sesavat-unperceived cause from perceived effect.
iii) Samanyatodrsta- when inference is based on the relation of co-existence.
According to another classification(method of ascertainment of vyapti) inference are of three types.
i) Kevalanvayi- agreement in presence, (like Mill’s method of agreement)
ii) Kevala vyatireki- agreement in absence (like Mill’s method of difference)
iii) Anvaya vyatireki- Agreement in presence and absence, (like Mill’s joint method of agreement and difference)