This chapter explores a first meaning of autism, that it is a psychiatric disorder defined in a diagnostic manual. We investigate what a psychiatric disorder is. I give an overview of philosophical discussions about the distinction between mind and brain, psychology and biology. I explain how biological explanations for mental disorders have the advantage that they can take away feelings of guilt and open up routes towards healing. At the same time I argue, using Karl Jasper’s distinction between understanding and explanation, that reductionist approaches that merely give a causal biological explanation of mental disorders neglect the importance of understanding the meaning and context in which phenomena occur.