For both theoretical and practical reasons researchers define emotions according to one or more dimensions. in his philosophical treatise, The Passions of the Soul, Descartes defines and investigates the six primary passions (wonder, love, hate, desire, joy, and sadness). Wilhelm Max Wundt, the father of modern psychology, proposed in 1897 that emotions can be described by three dimensions: "pleasurable versus unpleasurable", "arousing or subduing" and "strain or relaxation".[6] In 1954 Harold Schlosberg named three dimensions of emotion: "pleasantness–unpleasantness", "attention–rejection" and "level of activation".[7]