Silence, like a lot of other commodities, is a strange duck. Everyone just knows what it is, but in order to express what they know, they have to make quite a lot of noise.
Silence can be depressing -- as when no one seems to care about a wracking illness or a grinding poverty or a scraping loss.
Silence can be keeping your mouth shut, but when you examine it, that's not quite silent either.
There's the silence of the meditation hall, the silence of the graveyard, the silence of darkness, the silence of light, the silence of salt, the silence of ... well, the silence of anything at all.
When it comes to silence, I just know what I'm talking about, but do I?
In the army, I worked in a sound lab for a while, helping to edit language tapes that would be distributed to students as a means of learning their given languages. My job was to excise the swallows and hesitations and spit sounds that people making the tapes invariably emitted. Russian, Chinese, German, Urdu ... every speaker had such glitches.
And one day, the guy who ran the lab came into my small room and asked me to come with him. He placed me in a sound studio in front of a microphone. All he wanted me to do was to sit still and make no sound as he recorded it. What he was after was "studio sound" that he could later splice into language tapes where there were pauses that were needed. When I asked him why he couldn't just splice in blank tape, he said it would sound unnatural and interrupt the flow of the tape. He wanted the sound of silence. (I managed about 50 seconds in front of the sensitive microphone before I was overcome by the need to swallow ... but my boss seemed pleased: He seemed to know that silence was no easy matter.)
Silence is more than keeping my mouth shut, I guess, but that still doesn't say what silence is. Silence may be something I just know, but I don't seem to know what I just know very well: If I say what it is, I miss the boat and if I don't say what it is, I miss the boat. Just knowing or certainty is as odd a duck as silence, I guess.
And if silence leaves me flummoxed, how much better informed am I about sound? The intellectually adroit may say the two -- sound and silence -- come as a pair, like peas in a pod or yin and yang, but that's just intellectually adroit and fails to address the question of what it is.
Oh well, I guess it may be good enough for most to just know and not worry about such things.
But for those who slow down long enough to wonder a bit, I think the Zen teacher Ummon may have offered a pretty good pointer:
"When you can't say it, it's there. When you don't say it, it's missing."