A couple of days back I re-washed a dish which I felt was not clean enough to my liking. While doing that I thought of all the people out there who would not have bothered doing what I had done and then suddenly this struck me - whatever I was defining as cleanliness was still only a perception.

No matter how much I cleaned the dish, it would never become perfectly clean simply because it is physically impossible to get to that 100% clean state where the dish does not have any particulate residual matter left on it. Every iteration of cleaning would only reduce the absolute amount of residual matter left on the dish until the amount of particulate matter in the water used for cleaning the dish becomes the limiting factor to the cleaning process.

The process is similar to trying to displace a liquid, say ethanol, from a container through a hole in its bottom by continuously pouring another miscible liquid, say water, into the container. No matter how long you pour the second liquid the concentration of the first liquid in the container will never become absolute 0. It will only keep on decreasing infinitely.

As with the above example, when you say that something is clean it only would mean that the levels of unacceptable matter (organic or inorganic) on the object is below a certain allowable threshold ppm thus making the definition of cleanliness an implication of the concentration of the unacceptable matter on the object and never an absence of the unacceptable matter.

You can extend this to your hands also. If you touch something dirty, it is going to take quite a lot of washing to really take the dirt of your hands. That would bring in the importance of washing with soap as it would try to chemically remove/neutralize the biotic and a-biotic waste on your hands.

So next time you worry about cleanliness, think of it, it is all relative and nothing is perfectly clean :-).