Asama Vyapti is a philosophical term suggesting the relationship between two terms having unequal extensions. Vyapti or invariable concomitance of two terms is the logical necessity for the possibility of inference, according to the Nyaya School of philosophy. The two terms of vyapti are hetu and Sadhya; hetu serves as the logical ground for inferring Sadhya. The Nyaya concepts of hetu, Sadhya and vyapti come close to the western logical concepts of middle term, major term and the major premise respectively.
Vyapti signifies the invariable pervasion of hetu by the Sadhya
term. That is, all instances of the presence of hetu are necessarily instances
of the presence of Sadhya as well. As such, hetu is always vyapya (the pervaded
term), whereas Sadhya is always vyapaka (one that pervades). It is always the
presence of the pervaded term that provides the logical ground for inferring
the presence of the vyapaka term.
In the classical Nyaya example of vyapti of smoke and fire,
smoke is the pervaded term (vyapya) and fir the vyapaka term. Therefore, the
presence of smoke leads to the inference of fire and not vice-versa. The impossibility
of inferring smoke on the basis of fire is due to the fact that all case of
fire are not necessarily cases of smoke as well. That is, the extensions of
fire and smoke are unequal. The term whose extension is pervaded by or
contained within the extension of the other term is called hetu or vyapya and
it serves as the grounds for inferring the other term, called Sadhya or
vyapaka. Such vyapti allows unidirectional inference only.
Thus, a vyapti which involves terms of unequal extensions is
called asama vyapti or visama vyapti. It is also known as non-equipollent
invariable concomitance. The other possible kind of vyapti is sama vyapti or
equipollent invariable concomitance in which both the terms have an identical
extension. Thus, in case of sama vyapti either of the terms can serve as hetu
or inferring the other term.