Nyāya or logic is an important aspect of all philosophical systems. It is needed either to prove one’s standpoint or to contradict and disprove that of the opponent.
One of the means of knowledge—called ‘pramāṇas’ or valid sources of know-ledge—is anumāna or inference.
The most frequently quoted and widely accepted mode of expressing the anumāna pramāṇa is as follows:
There is fire on the yonder hill, because smoke is rising from there. It is well-known from our experience (of direct perception) that wherever there is smoke, there is fire. Hence, by seeing the smoke, eventhough we may not be actually seeing the fire, we can infer the existence of fire there.
In this example, the rising of smoke which is invariably associated with fire, is technically called ‘hetu,’ ‘liṅga’ or ‘sādhana’. When this hetu is only an ābhāsa (illusory), it is called ‘hetvābhāsa’. For instance, take this statement:
‘All bipeds are rational.
Swans are bipeds.
Hence, swans are rational.’
The conclusion of this inference is false because the hetu given (that all bipeds are rational) is itself ābhāsa, illusory or unreal.
Hetvābhāsa is of five kinds: savyabhi-cāra, viruddha, satpratipakṣa, asiddha and bādhita.
See under each title for details.