What did Piet Mondrian once say was the foundation of all art?

Piet Mondrian believed that the line was the foundation of all art. He believed that lines had the power to create structure, rhythm, and harmony in a composition. By using lines, artists could define shapes, create movement, and convey emotions. Mondrian's own abstract art was characterized by the use of straight lines and primary colors, reflecting his belief in the importance of the line as a fundamental element in art.

Pieter Cornelis Mondriaan (7 March 1872 – 1 February 1944), after 1906 known as Piet Mondrian, was a Dutch painter and art theoretician who is regarded as one of the greatest artists of the 20th century. He is known for being one of the pioneers of 20th-century abstract art, as he changed his artistic direction from figurative painting to an increasingly abstract style, until he reached a point where his artistic vocabulary was reduced to simple geometric elements.