Piet Mondrian once stated that what type of line was so emotional?

In Amsterdam in the 1920s. The leading artists in the group 'De Stijl' were Piet Mondrian and Theo van Doesburgh. Mondrian had written about the aims of the group: 'only primary colours and non-colours, only squares and rectangles, only straight and horizontal or vertical lines'. van Doesburg advocated less strict rules, and the quip about emotional was Mondrian's rebuttal.