abhAva
abhAva is the negation of an entity. It is defined as prathamapratItau yaH nAsti iti upalabhyate saH abhAvaH - tattva sa~NkhAnam. The one which is perceived as non existent when it is first perceived is called abhAva. For ex. nAstyatra ghaTaH is an example perception of an abhAva. Here the abhAva is called ghaTAbhAva as it is the negation of ghaTa (pot).
abhAva is of of three types (prAgpradhvaMsasadAtvena trividho.abhAva iShyate - T.Sankhyana)
Polemics:
Some argue that there is no entity called abhAva as every entity necessarily falls under the category of bhAva. To this we ask, what is the object of the perception "nAstyatra ghaTaH - There is no pot here". According to the arguer, it cannot be abhAva, as he is debating its very existence. So, he should say, the object of this perception is also bhAva only. So, what is the object of this perception? We have two options here
1. Only the bhUtala (ground)
2. bhUtala along with some other bhAva object.
The first case has a flaw. If the object of the perception "nAstyatra ghataH" is only the bhUtala then whenever we see bhUtala we should get the perception "nAstyatra ghataH". This should be the case even when "the pot" is present on the ground. This leads to contradiction. As "the pot" and "absence of pot" cannot exist on the ground at the same time.
In the second case too, if the other bhAva object is the "pot" itself then it leads to the same contradiction. On the other hand, If the bhAva object is something else other than pot, then it leads to atiprasa~Nga - farfetched conclusion. Consider a pot having two attributes rUpa(shape) and gaMdha(smell). When someone perceives the rUpa of a pot then normally he should get the perception of the kind "rUpam asti". But if we agree to the logic that we can get an abhAva pratIti (perception of negation) even when the object of the perception is a bhAva, then we may get the perception "gaMdho nAsti" - "there is no smell" just by perceving the rUpa(shape) of the pot. It is summed up as rUpavati ghaTe gaMdho nAsti iti pratIti prasa~NgaH.
Therefore it is not correct to say that the bhUtala or the ground is the object of the perception of the type "nAstyatra ghataH". By process of elimination, it is established that no bhAva can be an object of such a perception. Therefore we should agree than an abhAva is the object of this perception. To differentiate one abhAva from the other, we have to accept that for the perception "nAstyatra ghataH" to occur, the object should be ghATAbhAvaH.
See Also