There are essentially three sources of valid cognition: direct perception (pratyaksha), inference (anumana), and valid testimony (shabda). Of course, the quality of all three can be compromised by our own or others’ afflicted emotions and obscurations to wisdom.
To give some common examples:
When we are in a hurry, and looking for our car keys, we may be searching everywhere for quite some time, without realizing we have them in our hands. In this case, our organs of perception are not impaired and are indeed working properly, but our mental consciousness does not take notice. Or the percept (object) may be beyond the range of our perceptive capacity (i.e., not within the segment of the light spectrum we can see, or outside the range of our hearing, etc.). These are examples of compromised direct perception.
A common form of compromised inference is to assume that correlation equals causation, as when data ‘establish’ that the per capita income in a particular location is higher than in another, and we conclude that even the poor in the first location are better off than their counterparts in the second. Without knowing the distribution of wealth and the cost of living in locations A and B, the inference is not valid.
If we ask a witness for testimony on a particular event, (1) if this individual’s perception is impaired, (2) if their inferences are invalid, or (3) if they willfully lie, reliance on that (invalid) testimony leads to compromised cognition.
When relying on the testimony of another’s direct perception, that reliance must always be tempered with the understanding that (1) the testimony is only as good as the witness is credible, and (2) our own understanding of that testimony is only as good as the clarity of our minds allows.
However, when one has repeated experience of the reliability of particular witnesses —Shakyamuni, Maitreya, Asanga, Vasubandhu, Nagarjuna, Dolpopa, Taranatha— one will give preference to their testimony over the statements of others. This is often confused with so-called ad-hominem arguments, but is a sound logical procedure. Even worldly courts weigh more heavily the testimony of reliable witnesses than those of others, even when the testimony of the latter seems plausible.
When we rely on valid testimony (shabda) —i.e., Dharma teachings pointing at truths of which we may not (yet) have direct perception (pratyaksha)— such testimony may have been originally established through:
1. direct perception (pratyaksha),
2. valid inference (anumana), or
3. the testimony (shabda) of a previous Dharma Teacher.
In the first instance, the Teacher who presents the valid cognition to us has directly perceived it. In the second, the Teacher has correctly inferred such valid cognition from established facts. In the third, the Teacher is repeating a valid cognition that has been transmitted by a valid cognizer, i.e., a previous Teacher who has, in turn, directly experienced, correctly inferred, or purely received that transmission of the valid cognition.