  All the classical schools that advance epistemologies accept perception as a knowledge source although there is much disagreement about its nature, objects, and limitations. Are the objects of perception internal to consciousness or external? Are they restricted to individuals, e.g., a particular cow, or are universals, e.g., cowhood, also perceived? How about relations? Absences or negative facts (Devadatta’s not being at home)? Parts or wholes? Both? A self, awareness itself? There are issues about perceptual media such as light and ether, ākāśa, the purported medium of sound, and about what is perceptible yogically (God, the īśvara, the ātman or self, puruṣa). What are the environmental conditions that govern perception, and how do these connect with the different sensory modalities? Are there internal conditions on perception (such as attention or focus, viewed by some as a voluntary act)? Is a recognition, e.g., “This is that Devadatta I saw yesterday,” perceptual? And does it prove the endurance of things over time including the perceiving subject? Do we perceive only fleeting qualities (dharma), as Buddhists tend to say, or qualifiers as qualifying qualificanda (a lotus as qualified by being-blue), as say realist Nyāya and Mīmāṃsā? Does all perception involve a sensory connection with an object that is responsible for providing its content or intentionality (nirākāra-vāda, Nyāya), or is the content of perception internal to itself (sākāra-vāda, Yogācāra)? How do we differentiate veritable perception, which is defined as veridical, and pseudo-perception (illusion), which is non-veridical? How is illusion to be explained? These are some of the outstanding issues and questions that occupy the schools in all periods of their literatures.