Various darshanas accept shabda as a svatantra pramaNa. Trustworthy statements found in shAstra, purANa and itihAsa generate a certain knowledge which cannot be classified as either pratyakSha or anumAna. That thus gets classified into a separate pramANa named shabda. However, every shabda does not qualify as a pramANa. Gautama defines shabda pramANa thus:
AptopadeshaH shabdaH [nyAyasUtra 1,1,8]
vAtsyAyana explains this sUtra in great detail, defining Apta as a person who has directly realized the topic in discussion. Such a person, for the sake of lokopakAra, states the anubhavasiddha viShaya; such statements alone qualify as shabda pramANa.
aNNaM bhaTTa defines Apta thus:
Aptastu yathArtha vaktA |
An Apta is one who states simply what he sees or experiences. Only his words can be accepted as prAmANika.
shabda pramANa is of two kinds:
What can be seen by oneself in the world around is termed as dR^iShTArtha. For example, the statements made in jyotiH shAstra regarding grahaNa can be seen and verified by all. Such pramANa is also termed as laukika vAkya.
When the statements are in reference to paralaukika viShaya and not laukika, such pramANas are called adR^iShTArtha. artha siddhi of vedic verses does not lie in the realms of laulika or pratyakSha and hence are included in this second category. Gautama points out that the same seers who have given us dR^iShTArtha vAkyas have also revealed adR^iShTArtha vAkyas. As the former gets accepted as pramANa, there should be no harm in accepting the latter as statements of both kinds have the same origin:
mantrAyurvedapramANyavaccha tatpramANyamAptaprAmANyAt |
It is sufficient to examine a single grain of rice to determine if an entire pot of rice is cooked. Following the same sthalIpulAka nyaya, after verifying the validity of some AptavAkyas, the satyatA of the rest can be inferred.
vaidika vAkya is again of three kinds:
1. vidhi vAkya: Aj~nAsUchaka or illustrates an order. For example: agnihotraM juhuyAt svargakAmaH |
2. arthavAda: This is again seen in four forms:
a. stuti vAkya: Glorifies vihita karma through a description of iShTa phala. For example: By performing Yajna A, one conquers the devas. The purpose of such statements is to inspire the listener so as to bring about pravR^itti in that karma.
b. nindA vAkya: Reproaches the niShiddha karma by showcasing the aniShTa phala. For example: By not performing Yajna B, one attains naraka for eight thousand years. The purpose of such statements is to bring about nivR^itti from such karma.
c. prakR^iti vAkya: Illustrates the contradiction within manuShya kR^ita karma. For example: Some offer Ahuti thus whereas others offer it differently.
d. purAkalpa vAkya: States aitihya or vidhi that is handed down through paramparA. For example, great sages have practiced this vidhi thus, we should follow the same.
3. Mantra: A declarative statement.
That a mantra can be vidhi as well as arthavAda is something that has been dealt with by various mImAmsakas and by our very own bhAskararAya.