  Appealing to levels of explanation in psychiatry, then, might just be a reminder that we need to relate variables of many sorts in explaining the causes of disorder. But it may also be connected with particular perspectives on reduction, explanation and causation. We won’t go into these debates here; rather, we will move on to discuss a further problem. As well as great diversity in causal factors, psychiatry also confronts the problem of diverse symptoms; many mental illnesses take a different form in different people. Although two people might both be diagnosed with major depression, they need share few symptoms. Both are very likely to feel miserable and lose interest in activities they normally enjoy. However, one might lose weight, sleep less and becomes physically agitated while the other sleeps more and becomes lethargic and tormented by feelings of guilt and thought of suicide. The philosophical problem arises from the confrontation between the great variation in clinical reality and the need to simplify in order to render that reality scientifically tractable. In summary form, the problem is; what are we trying to explain when we explain a mental illness? Is it a diverse set of real-world phenomena, or some simplified representation of those phenomena?