This is the quick answer to this fundamental question. The more likely we are to encounter that system, the more fit it is.
Fitness is the most important and tricky term of the answer to define.
It can only be defined in terms which are not obvious themselves, and so need further definitions, and so on. One can hope that after a few rounds of definitions, the meaning will become sufficiently intuitive to be satisfactory for most readers. At the very least, we assume that there exists a partial ordering, i.e. some configurations are more fit than others. Increase of fitness determines a preferred direction of evolution . The argumentation for this can be found in the variation and selection principles of evolution .