We plan and produce objects of various sorts in order that they do something specific. The function of a man-made object is then, it seems, merely the particular effect for which it has been made.1 It is more demanding to justify the answers concerning the biological item. One needs to make sense of the idea that nature may pick out a particular effect and turn it into a function. As is well known, one way to do this is by employing the selectionist etiological theory of functions, SEL for short. This theory, proposed by Millikan, Neander and others in the 80s, identifies the function of a biological trait with its selected effect or the effect which, pushed by natural selection, explains the diffusion or the conservation of the trait in the population (Millikan 1984, Neander 1991). So, when we begin to investigate the nature of functions, two different sorts seem to emerge depending on whether we consider artifacts or natural entities: on the one hand there are those that result from human intentions while on the other hand there are those that are due to natural mechanisms like natural selection. Upon closer analysis, however, the artifact case proves to be more complex.