One thing you learn with any kind of mathematical or science background is that your measurements can only be as good as your measuring instrument. If you you are using a common scale - you cannot very well say something is 1.623 mm.
But just because you cannot measure it doesn't mean it does not exist. Get me a vernier caliper or a micrometer and I could measure it down to to those levels.
Now apply the same principle to cleaning. You've washed, dusted, vacuumed, and generally cleaned the heck out of your living room. So there is no more dust or dirt, right ? wrong ! Get me a microscope and I'll show you the amount of dirt still there.
All you have done is to clean the place to the level where the dirt and the dust particles are below your threshold of visibility. But just because you cannot see them does not mean they do not exist !
Its also a good lesson in life. Just because we stop noticing something does not make it go away. If anything is important enough to matter, we need to break out our calipers and start measuring it - because we cannot make it go away just by ignoring it.