the main problem for action selection is complexity. since all computation takes both time and space (in memory), agents cannot possibly consider every option available to them at every instant in time. consequently, they must be biased, and constrain their search in some way. for ai, the question of action selection is "what is the best way to constrain this search"? for biology and ethology, the question is "how do various types of animals constrain their search? do all animals use the same approaches? why do they use the ones they do?"