  The idea here is that the possibility of recognitive experience presupposes the existence of a unitary enduring self. There are really two arguments compressed here. One is this: I cannot remember an object unless I have seen it earlier—I cannot remember what you have seen (cf. Locke’s account of personal identity in terms of a continuity of memory experiences). The other argument is more intricate. The Abhidharma Buddhist claims that a person is just an aggregate of experiences, some visual, some tactile, etc. The problem raised for such an account by the Naiyāyika is that it is possible to make trans-modality judgements, in which the deliverances of one sense-faculty are compared with another. Suppose that V(o1) is a visual experience of an object o1, and T(o2) is a tactile experience of an object o2. It is possible to judge “o1 = o2”, but this judgement does not belong to the sphere of any one sense-faculty.  Hence, it must be located in some other substance. The self is here conceived of as that which collates, organises and compares the deliverances of the individual senses. See Ganeri 2012, chapters 11, 15, 16 for an expanded discussion.