Also, his abstract art was a push back against the realism in art at the time plus the horrible realism of war. He family said, "Abstract art is not the creation of another reality but the true vision of reality". He believed that by abstracting forms and colors to their essential horizontal and vertical lines and primary hues, an artist could express the fundamental, spiritual truths of the universe and achieve a pure, universal beauty and dynamic equilibrium.

Source: I've done a lot of Mondrian research while creating a board game where you actually make art in the Mondrian style while playing.