Levels of description

I am a big fan of Douglas Hofstadter. One of the most profound and beautiful ideas I encountered while reading the books of Douglas Hofstadter is that of looking at phenomena at different levels of abstraction and the emergence of pattern.
The first thing to note, which is somewhat trivial but we don't often spell it out explicitly, is that many phenomena in nature can be perceived at varying granularities. A human being for example can be viewed as a single atomic entity, with rules(social/psychological) dictating it's behavior. This is the level in which we are used to relate to ourselves and the rest of the people around us. Beneath this level there is the biological level, which is the view we might switch to when we are ill while that upper level is insufficient. In that view an individual is a collection of smaller functional units (either organs or in an even lower level a bunch of cells). Beneath it are the chemical level and the atomic level and so on... What is the correct level to describe a human being? What does "correct" mean in this context?
Indeed the question needs context, and I would replace "correct" with "useful" as well. As mentioned, in everyday life we usually have no need for for the lower levels and so a sufficient understanding of our social surrounding can be obtained by our high level model of human beings. Why is it useful? well think about trying to predict your friend's behavior (how is he going to react to you telling him you just bumped into his car in the garage?) from the physical level? You would need to simulate in your head the interaction of billions of friend-atoms, updating their state after each time frame, for millions and millions of frames. I assume we would need brains the size of football stadiums to do these calculations.
The beauty of it is that many situations do not require going lower. Despite the strong dependence of the levels , there is a sense in which they are completely independent of each other. You could completely replace the lower level with another and the higher level would stay unchanged.
This is a truly marvelous idea and I always enjoy watching the way it manifests itself in Conway's Game of Life. Here is an example:

On the low level you have the simple rules of the game and the square grid elements. On a higher level (which our mind always prefers) you start seeing other patterns emerge. We start talking about gliders, spaceships, boats etc... these higher elements have their own rules to how they conduct themselves in this world. Higher level elements are not special to the exact rules which Conway chose (in spite of him choosing them quite selectively). There are modified rules which bring about similar spaceships moving around the world in a similar manner. This is indicative of the independence between the levels of abstraction.