Any object that a cognition cognizes exists in the external world; or, any object that a cognition cognizes does not exist in the external world. These are the alternatives Prajñakaragupta gives in his Alamkara. Buddhist idealists use an illusory cognition, such as a dream cognition, as an example to illustrate the latter. But can we conclude, from the fact that such an illusory cognition cognizes an object which does not externally exist, that any cognition has no externally existent object? This is the problem of generalization Matilal (1972) pointed out concerning the “Argument from Illusion” of Buddhist idealists. Prajñakaragupta tries to solve this kind of problem from the viewpoint of the theory of self-cognition (svasamvedana), showing that the problem can be solved by Buddhist idealists, but not by realists.
Kumarila, a typical realist, holds that it is what exists in the external world that brings about an illusory cognition. For example, in the case of the mirage illusion, the appearance of water in this illusion presupposes the following two things: the water has been previously seen; and, sands heated by the sun are present.
Prajñakaragupta, a Buddhist idealist, criticizes this view of Kumarila's by saying that there is no way to know the existence of things that do not appear to a cognition. According to Prajñakaragupta, if we see an object other than what we are seeing, then we would have the cognition of yellow when we see blue; therefore, what we are seeing is just what appears in the mind.
It is to be noted that in the view that every cognition has a general character, i. e., the self-revealing character, the problem posed by Matilal is easily solved. For, there is no difference between the water which appears in an ordinary cognition and that which appears in a mirage-illusion, because both appear in the same form of ‘water.’ On the contrary, if the realists insist that there appears the external object even in the illusory cognition, they have to clear up the problem: How is it that only through an ordinary cognition we see what we are seeing now and here, and not through illusory cognition?