In the present work we attempt to analyze one of the least known, but most important, aspects of radical behaviourist philosophy and behaviour analysis: the scientific approach to the facts, phenomena, processes, behaviours, etc. that can't be observed by another organism (private events).For any discipline dealing with their study, these phenomena raise several problems at different levels. Ontological, epistemological, theoretical, methodological and applied issues can be found. Throughout our article we will describe the responses to these problems provided by radical behaviourism and behaviour analysis in the historical context of behaviourism. Finally we will compare this account with the one proposed by cognitive psychology.