Aptavakya are words of a reliable person, of one who speaks what he sees. Ten sources of knowledge are recognized in the Hindu philosophical system. Sabda, translated as verbal testimony, is one of them. Sabda, when it serves as a source of knowledge, is called aptavakya. This sabda is of a two-fold nature: the words of the Vedas and the words of other texts or persons.
The Naiyayikas who accept the validity of Vedas do not
accept them to be apaureshya (non-contigent) and nitya (self-existent).
Akshapada Gautama, author of Nyaya aphorisms, has defined sabda as aptopadesah
(instructions of reliable persons), paraphrased in later literature as aptavakyam.
The word apta is interpreted as sakshakrtadharma (one who has seen/known the thing
himself). Such a person may desire to communicate what he has seen/experienced.
He may not do so owing to lack of interest/sympathy or laziness or communicate
wrongly out of jealousy.
These are described as bhrama, pramada and vipralipsa (wrong
knowledge, carelessness and desire to mislead/cheat). The word upadesha (verbal
instruction) also introduces as physical element into it. Vakyam of later literature
makes this physical element explicit. Thus, if any wrong understanding results
from a communication because of defective limbs, though the intention to cheat
may not be there, then it is communication only and not upadesha (verbal
instruction) of what is sought to be conveyed.
Knowledge of ideas is to be secured through sudrdhama
(proper and unquestionable means of knowledge). It should avoid what is not
good. An apta is one who is moved to communicate what he has convincingly
known; he should rise above likes and dislikes, favor and hatred and as such is
detached. Hence an apta is a Vitaragah, one who has given up desires.
Communications which turn out to be anonymous – knowledge handed
down traditionally without the source being known – and convey correct knowledge
of the good and the bad are also considered as aptavakyas.