In this chapter, the problems about single events are discussed. The frequency interpretation has a major demerit that it cannot account for the probabilities assigned to single events. It essentially involves references to sequences of events, repeatable experiments, and the like, but it seems very questionable that such sequences can be found in the case of single events, too. On the logical interpretation which constructs the system of IL-language and takes probability as a logical relation between evidence and hypothesis formulated in the language, it is rather difficult to construct languages that can formulate various evidence about a single event, and to decide which confirmation function we should choose. The subjective interpretation has no such difficulties, for it concerns with single facts only, and does not need to construct any language system. So, it is necessary to examine the definition of probability itself here, which leads to the investigation of premises included in it. Since subjective probability is an operational notion, from this point of view its premises are criticized.