Regarding the definition of the concept of delusion it is stated that up to now delusions can be diagnosed only with the aid of phenomenological intuition. The decisive criterion is the high subjective certainty in the sense of an elimination of the possibility of chance coincidences without any narrowing of affect. The current hypotheses are presented, concerning the choice of theme, the acute elimination of chance without narrowing of affect, the persistence of certainty and incorrigibility. The relationship between age at the onset of the illness and the choice of theme are discussed in the light of my own findings. Reasons emerge to believe that, depending on the respective cultural setting, certain age specific themes can play a role in the formation of the content of delusions. The acute subjective certainty, as well as the incorrigibility are linked with organic or endogenous syndromes.