I don't know how we sense it...that thing, be it the big thing, or the little thing, or the overwhelmingly present thing that lurks just out of view, but we do. Some people call it instinct, but that endows the very nature of the beast with a questionable mythology, when I tend to think of it as simply a different form of sensitivity.
It seems to me we are capable of sensing discordiant presence on the microlevel and processing it on a meta level. We can see, smell, taste or touch something and feel the piece suddenly missing on a subconcious level, even if the conscious eye misses it on first scan.
The real question then becomes: what to do about it and how to find it. How do we find the out of tune instrument in life's orchestra? How do we catch and fix the errant voice in our inner chorus?
Because the thing is we have to, or it will be there, irksome and imperfect, ruining the beauty of every rise and fall, every rhythmic triumph tarnished by the mistakes and errors of this one errant piece. And soon, we wont hear the music, we'll only hear that one damn voice.
Now thats okay. Because that voice cries to be heard. The trick is hearing what it has to say before it turns to a dull scream.