Levels of reality and scales of application

Philosophers and scientists often describe theories, laws, and explanations as applying to the world at different 'levels'. The idea of a 'level of application' is often used to demarcate disciplinary or sub-disciplinary boundaries in the sciences. For instance, stoichiometric laws and quantum mechanical laws might be said to describe chemical phenomena at different levels. More generally, the idea of levels is used to distinguish more fundamental laws or theories from less fundamental ones: more fundamental theories are those that apply at more fundamental levels.