What precisely does aristotle mean when he asserts that something is (or comes to be) "for" "the" "sake" "of" something? I suggest that the answer to this question may be found by examining aristotle's position on the problem of reduction in biology, As it arises within his own scientific "and" "philosophical" context. I discuss the role of the concepts of "nature" and "potential" in aristotelian scientific explanation, And reformulate the reduction problem in that light. I answer the main question by (...) establishing that aristotle holds an "irreducibility" thesis in regard to the generation and development of a living organism, And that this thesis is the core of his conception of final causality. I conclude by arguing that aristotle's teleology is fundamentally "empirical" in character, And not an a priori doctrine brought "to" the study of nature. (shrink)