The sources of right knowledge are direct perception, inference and scriptural testimony.
Comprehension is based on direct observation of the object, inference, and reference to reliable authorities.
The first translation above is from Sri Swami Satchidananda; he’s my main source for the sutras. The second translation is from T.K.V. Desikachar. In this case, I find Desikachar’s slightly different choice of words really helpful.
We’re still talking about vrittis here, mental modifications, which we want to control or restrain so the mind can become quiet. One of these vrittis is “right knowledge” or “comprehension.” The things we know or believe to be true define our worldview. Learning to control the vritti of comprehension means seeking truth and weeding out falseness. Like everything in yoga, this is a constant process.
Direct perception and inference are pretty self explanatory. If we experience something first-hand, we believe it to be true. When we encounter a similar situation later, we may have a reasonable expectation about the outcome based on inference. Both of these can still lead to incorrect conclusions because of any number of factors that we can’t predict or control. That’s why we have the third source of knowledge, scripture or reliable authorities.
Even if you’re not a religious person (and I’m not), it’s worth acknowledging that there are some good reasons religion and scripture stick around for so long. There’s a great deal of wisdom to be gleaned from teachings that have stood the test of time. However, we’re talking about one external source of knowledge here versus two internal ones, and real wisdom seems to be a mashup of all three.
A person can never be wise if they distrust their own understanding. Our first source of knowledge is experience. Inference flows naturally from experience. If I have experience x, then y must be true. Simple. But because we can’t know all the factors in many situations, our inferences can be unreliable, and we turn to our trusted sources. Scripture is probably closest to the word Patanjali would have used in English, but I think science belongs in the same category in the sense that both are based on the best knowledge humans have at a given time.
In this sutra, Patanjali asks his students to find balance between internal and external forms of knowledge. Find the intersection where your personal experience, your own logic, and the wisdom of trustworthy authorities meet. That’s where you’re most likely to find truth.