Clinical Philosophy (CP) can be understood as an approach to the treatment of mental disorders. The goal of the present study is to put this CP approach to a first empirical testing. I present Arnold, a comorbid case with an insufficient treatment response to conventional, standard cognitive-behavioral therapy (CBT). Using a single case pre-test/post-test design a CP approach was developed drawing heavily on existentialist and philosophy-oriented writers. The client responded well to this novel treatment approach. Above all, levels of intolerance of uncertainty improved greatly from pre- to post-treatment. Also, a decrease in overall illness severity as well as specific psychopathological variables such as obsessive-compulsive symptoms, depression, and anxiety could be observed. Results are discussed in terms of the underlying mechanism of the CP approach. An account of the underlying mechanism of efficacy, understood as a tripartite function, is introduced. CP as a philosophy-oriented method within the broader framework of third wave CBT and existential analysis is considered.