How do we perceive the world? Based on knowledge concerning our perception system, it has been concluded that what we experience subjectively is not the result of direct sensory input, but a construct of our brain. This construction system, an internal world model, is used to achieve a consistent (not necessarily true) interpretation of the world, necessary for both sensing and acting. This system uses sensory data as well as innate mechanisms and information from memory, that is, it comprises a cooperation of bottom-up and top-down mechanisms. The mechanisms underlying this cooperation follow the principles of selectionism. This idea allows us to understand how categories can occur as private, 'subjective" phenomena. Shared categories are necessary, however, to achieve "objective" phenomena. In fact, the selectionist mechanisms automatically lead to the development of similar categories in different individuals, with categories being similar enough to enable communication between individuals. This means that objective phenomena form a subset of subjective phenomena. In other words, the terms objective and subjective do not represent separate ontological entities. Objective phenomena are subjective phenomena that can be characterized by a small number of categories.