A tentative classification of transactions of living organisms with the environment is presented. The classification is based on the complexity with which spatial and temporal factors are dealt with, particularly with respect to anticipatory behavior, by different phylogenetic levels of the central nervous system. Subjective phenomena are discussed and it is proposed that 4 major classes of such phenomena be differentiated, related either to activities in certain areas of the nervous system or to modes or particular patterns of interaction with the environment. It is noted that phenomena of all 4 classes may occur simultaneously and modify the immediate subjective impression.Abstract Teaser