According to statistics one in five people experiences depression. If depression is so common, how do we know it is an illness and not just a normal part of being human?

That is an excellent question. The distinction between health and illness is tremendously controversial. Some philosophers believe that the difference is fixed entirely by various facts about the natural world. These philosophers might point out that insofar as depression arises from the production of certain extreme quantities of some neurotransmitter or from some particular gene, which ultimately inhibits or prevents certain cells from carrying out their basic life functions (e.g., from employing a certain metabolic pathway to derive energy), then depression has a biochemical basis. On this view, that depression is common does not change the fact that it involves the malfunctioning of some part of the body, where "malfunctioning" can be cashed out in exclusively naturalistic terms. (But what, then, does it mean for a part of the body to function properly? What determines the body part's biological function? That is a controversial question.) Other philosophers disagree. They believe that the...