 That said, most of Aristotle’s readers do not find themselves in need of a defense of the existence of efficient causation.  By contrast, most think that Aristotle does need to provide a defense of final causation. It is natural and easy for us to recognize final causal activity in the products of human craft: computers and can-openers are devices dedicated to the execution of certain tasks, and both their formal and material features will be explained by appeal to their functions. Nor is it a mystery where artefacts obtain their functions: we give artefacts their functions. The ends of artefacts are the results of the designing activities of intentional agents. Aristotle recognizes these kinds of final causation, but also, and more problematically, envisages a much greater role for teleology in natural explanation: nature exhibits teleology without design. He thinks, for instance, that living organisms not only have parts which require teleological explanation—that, for instance, kidneys are for purifying the blood and teeth are for tearing and chewing food—but that whole organisms, human beings and other animals, also have final causes. 