This paper presents Niketas Stethatos’ use of the definition of philosophy as “knowledge of human and divine things”. The definition, of Stoic origin, was elaborated by the Neoplatonic school of Alexandria (Ammonios, Elias, David, Pseudo-David) together with five other definitions, and was adopted by the Church Fathers. The first part discusses aspects of the definition’s history in ancient, Patristic, and Byzantine literature until the eleventh century, which indicates Stethatos’ uniqueness. The second part presents the definition in his works (phraseology, sources and use), with emphasis on its relation to the φυσικὴ θεωρία. Also the extraordinary often and multifaceted appearance of this definition in Stethatos’ works in his controversy with Michael Psellos is investigated. Stethatos, through his elaborated and systematic reference to the definition, propagates the mystical experience as authentic philosophy against the rise of secular philosophy and its tendency to become autonomous.