Endell's articulation of this new way of seeing and of the psychological power of form brought this aspect of Jugendstil to theoretical clarity in a way that was immediately and profoundly influential. His widely published works and writings, as well as his teaching, belong to the prehistory of Bauhaus aesthetics. Peg Weiss has demonstrated, for instance, the significance of Endell's writings for the theories of Kandinsky, through whom empathy came into expressionism in both its prewar agonistic and postwar revolutionary-utopian phases. Psychologized formalism became what Klee described in 1928 as "the science of art, including the unknown quantity 'x'," a science brought to the United States by figures such as Gropius and Albers, who Hejduk cited as influences at the end of the 1940s, as well as Rudolf Arnheim, whose writings systematized earlier experimental work on "visual thinking."
Believing that color theory was already well-developed by painters, Endell drew on the optical experiments of Lipps to devise a theory of the emotional effects of form. Lipps's well-illustrated book described the different effects of points, lines, and forms, illusions of relative size and directionality, dynamic as well as static properties, and above all the capacity of forms to induce feelings of expansion, contraction, tension, and rhythm. For Endell, feelings associated with qualities such as warmth or coldness, heaviness or lightness, the terrifying or the energetic could be expressed through tempo and tension and then translated into lines and complexes of lines, in a way that would parallel the expression of feelings through tonal complexes in music. He used architectural examples to show bow the manipulation of simple elements and proportions could produce varied emotional responses. A tall thin house with narrow vertical windows was interpreted as "not sympathetic," too hard and busy. In contrast, a low horizontal house with horizontally emphasized windows seemed "almost too comfortable." A facade with a balanced combination of forms, however, was considered to have "mild energy" and a "peaceful security."32
32. August Endell, Um die Schönheit, op. cit., 25.