"Abhava" (अभाव) means non-existence. Some scholars consider "Anupalabdi" to be same as "Abhava", while others consider "Anupalabdi" and "Abhava" as different. "Abhava-pramana" has been discussed in ancient Hindu texts in the context of "Padārtha" (पदार्थ, referent of a term). A "Padartha" is defined as that which is simultaneously "Astitva" (existent), "Jneyatva" (knowable) and "Abhidheyatva" (nameable). Specific examples of "padartha", states Bartley, include "dravya" (substance), "guna" (quality), "karma" (activity/motion), "samanya/jati" (universal/class property), "samavaya" (inherence) and "vishesha" (individuality). "Abhava" is then explained as "referents of negative expression" in contrast to "referents of positive expression" in "Padartha". An absence, state the ancient scholars, is also "existent, knowable and nameable", giving the example of negative numbers, silence as a form of testimony, "asatkaryavada" theory of causation, and analysis of deficit as real and valuable. "Abhava" was further refined in four types, by the schools of Hinduism that accepted it as a useful method of epistemology: "dhvamsa" (termination of what existed), "atyanta-abhava" (impossibility, absolute non-existence, contradiction), "anyonya-abhava" (mutual negation, reciprocal absence) and "pragavasa" (prior, antecedent non-existence).