The concept of mental disorder is often defined by reference to the notion of mental dysfunction, which is in line with how the concept of disease in somatic medicine is often defined. However, the notions of mental function and dysfunction seem to suffer from some problems that do not affect models of physiological function. Functions in general have a teleological structure; they are effects of traits that are supposed to have a particular purpose, such that, for example, the heart serves (...) the goal of pumping blood. But can we single out mental functions in the same way? Can we identify mental functions scientifically, for instance, by applying evolutionary theory? Or are models of mental functions necessarily value-laden? I want to identify several philosophical problems regarding the notion of mental function and dysfunction and point out some possible solutions. As long as these questions remain unanswered, definitions of mental disorder that rest upon the concept of mental dysfunction will lack a secure foundation. (shrink)