Attention Deficit Hyperactivity Disorder (ADHD) is a widely spread diagnosis. The dominant paradigm of ADHD is biomedical where ADHD is defined as a brain disorder. At the same time, the legitimacy of the diagnosis is being questioned since it is unclear whether or not ADHD can be deemed a medical disorder in itself. The aim of this article is to critically assess the merits of understanding the diagnosis of ADHD as a medical condition defined as a brain disorder. This is being done using the seventeenth century philosopher Benedict Spinoza's (1632-1677) notions of adequate and inadequate knowledge and his counterintuitive theory of mental health. Doing so it becomes clear that ADHD, however adequate it may seem, is founded on inadequate knowledge and that the legitimacy of the individual diagnosis should therefore be questioned on the grounds that on a long term scale it is passivizing and stigmatizing rather that liberating.