Various darśanas accept śabda as a svatantra pramaṇa. Trustworthy statements found in śāstra, purāṇa and itihāsa generate a certain knowledge which cannot be classified as either pratyakṣa or anumāna. That thus gets classified into a separate pramāṇa named śabda. However, every śabda does not qualify as a pramāṇa. Gautama defines śabda pramāṇa thus:
āptōpadēśaḥ śabdaḥ (Nyāyasūtra 1,1,8)
Vātsyāyana explains this sūtra in great detail, defining āpta as a person who has directly realized the topic in discussion. Such a person, for the sake of lōkōpakāra, states the anubhavasiddha viṣaya; such statements alone qualify as śabda pramāṇa.
Aṇṇaṁ bhaṭṭa defines āpta thus:
āptastu yathārtha vaktā |
An āpta is one who states simply what he sees or experiences. Only his words can be accepted as prāmāṇika.
śabda pramāṇa is of two kinds:
1. Dr̥ṣṭārtha
2. Adr̥ṣṭārtha
What can be seen by oneself in the world around is termed as dr̥ṣṭārtha. For example, the statements made in jyōtiḥ śāstra regarding grahaṇa can be seen and verified by all. Such pramāṇa is also termed as laukika vākya.
When the statements are in reference to not laukika, such pramāṇas are called adr̥ṣṭārtha. Artha-siddhi of Vedic verses does not lie in the realms of laulika or pratyakṣa and hence are included in this second category. Gautama points out that the same seers who have given us dr̥ṣṭārtha vākyas have also revealed adr̥ṣṭārtha vākyas. As the former gets accepted as pramāṇa, there should be no harm in accepting the latter as statements of both kinds have the same origin:
mantrāyurvēdapramāṇyavacca tatpramāṇyamāptaprāmāṇyāt |
It is sufficient to examine a single grain of rice to determine if an entire pot of rice is cooked. Following the same sthalīpulāka nyāya, after verifying the validity of some āptavākyas, the satyatā of the rest can be inferred.
Vaidika vākya is again of three kinds:
1. Vidhi vākya: ājñāsūcaka or illustrates an order. For example: agnihōtraṁ juhuyāt svargakāmaḥ |
2. Arthavāda: This is again seen in four forms:
a. Stuti vākya: Glorifies vihita karma through a description of iṣṭa 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 pravr̥tti in that karma.
b. Nindā vākya: Reproaches the niṣiddha karma by showcasing the aniṣṭa phala. For example: By not performing Yajna B, one attains naraka for eight thousand years. The purpose of such statements is to bring about nivr̥tti from such karma.
c. Prakr̥ti vākya: Illustrates the contradiction within manuṣya kr̥ta karma. For example: Some offer āhuti in a certain way whereas others offer it differently.
d. Purākalpa vākya: States aitihya or vidhi that is handed down through paramparā. 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 arthavāda is something that has been dealt with by various mīmāmsakas and by our very own Bhāskararāya.