Part 1. The possibility of instruction.
It is impossible for one mere human to directly infuse knowledge into the mind of another. Knowledge depends on a kind of light, and the teacher can only bring his students closer to receiving that light. He cannot directly transmit it, since he himself is not the truth.
There was a series of ads for PBS a while back based on the theme "Be More [insert adjective]". Each ad showed a scene in every day life which somehow demonstrated the exercise of some virtue. My favorite of these was the ad for "Be More Inspired", which communicates more about tradition and education in 30 seconds than I could in 10 minutes of talking.*
What I have always found most peculiar about this ad is that the virtue they think it represents is inspiration. The popular secular notion of inspiration has to do with witnessing awesome events or achievements in art: a space launch, a complex machine, a great book. Inspiration, for the secularist, comes from awe at facts of nature and especially at human works. This is not what the ad portrays at all. Instead of little children watching sprinters crossing the finish line or seeing a space shuttle launch, we have a man in an elevator talking. The entire ad focuses on people excitedly describing something to others around them. At first the speaker alone is suffused with the light of knowledge, but then the light ignites in each of his listeners, so that they go out into the world aflame with new insight.
By the secular mindset, the proper description of this ad would be "Be More Instructive", since the secularist can only admit that the light conveyed pertains to material facts about things. But for someone not crippled by the blindness of secular materialism — specifically, for someone who knows what it means to receive the Light into oneself and who has experienced the difference between living in that light and a life without it — it is difficult to see anything in this ad but a depiction of Pentecost. In this light it is clear that "Be More Inspired" is the perfect title: it not only adequately describes what's going on, but enriches our understanding of the whole.
So, back to the original point. Knowledge is a kind of seeing. Knowledge of what is taught comes not from seeing the teacher or hearing his words, but by learning to see what he sees, and being directed to this vision by means of his words and deeds. Hence learning is not learning about the teacher (except in extraordinary cases), but learning how to receive from reality the truth about things themselves. The teacher functions as a kind of sign directing the student's gaze toward the truth.
We should note, while we're here, that this insight about learning destroys one of the common hermeneutical problems of recent centuries. The question is how the teacher could transcend his own hermeneutical horizon (i.e. sphere of understanding) in order to transmit some specific intellectual content into the horizon of another. It's true; we can't do this. At the same time, the problem completely misses the nature of communication. Two people talking are not attempting to deliver their souls into each others hands. Rather they are engaged in a kind of outwardly directed synopsis or seeing-together. Their understanding of things is united by sharing the same real object. Thus this crisis about the possibility of intersubjectivity can persist only for the solipsist, who denies the existence of external reality.
"What was from the beginning, what we have heard, what we have seen with our eyes, what we looked upon and touched with our hands concerns the Word of life—for the life was made visible; we have seen it and testify to it and proclaim to you the eternal life that was with the Father and was made visible to us — what we have seen and heard we proclaim now to you, so that you too may have fellowship with us; for our fellowship is with the Father and with his Son, Jesus Christ. We are writing this so that our joy may be complete." 1 John 1:1-4
* [Note: The commercial I remembered did indeed have a striking title, but I was wrong on two points. First, it's called "Aura" and the tag-line is "Be More Connected", which makes little sense. Second, the thing begins with just the sort of nature video that I would have expected. Here's the link.]