According to Dharmakīrti, the particular (svalakṣaṇa) should be regarded as a real object of the valid means of cognition (pramāṇa) because it is the only thing that can cause an actual effect. Dharmakīrti claims that although inference (anumāna), which grasps svalakṣaṇa in another form, is erroneous, it can be considered valid for the reason that a cognizer, moving to something on the basis of inference, can obtain the effect that he expects. Inference as well as perception (pratyakṣa) is admitted as pramāṇa becasue it is non-deceptive (avisaṃvādin). Dharmakīrti illustrates his point by the jewel example: even if a cognizer mistakes the light from a jewel for an actual jewel, he can finally obtain a real jewel. What does this jewel example mean? Is the cognition of a jewel valid? If so, should it be regarded as a form of perception or of inference?
Indian commentators present differing interpretations of this example. From the context, Śākyabuddhi understands that the cognition of a jewel with regard to its light is a form of inference, i.e., inferring a cause from the observed effect. Śākyabuddhi thinks the cognition is erroneous, but valid. However, Dharmottara is severely critical of Śākyabuddhi’s view, and regards this as invalid cognition. Dharmottara suggests that even if the cognizer can finally obtain a real jewel in a room, it differs from the jewel that he falsely cognized as being at the keyhole of the door. Dharmottara does not acknowledge this cognition as inference. After Dharmottara, Prajñākaragupta insists that the cognition of a jewel should be regarded as perception. According to Prajñākaragupta, not only inference but also perceptions such as visual sensation must be erroneous, because neither can ever grasp their real objects that will be obtained in the future. In the conventional world, they are deemed valid simply because they are considered to be non-deceptive. Prajñākaragupta interprets that Dharmakīrti illustrates such conventional validity of pramāṇas by using the jewel example. Prajñākaragupta does not reject Śākyabuddhi’s view, but criticizes that of Dharmottara.
