Sauer defined the landscape as the defining unit of geographic study. He saw that cultures and societies both developed out of their landscape, but also shaped them too. This interaction between the natural landscape and humans creates the cultural landscape. Sauer's work was highly qualitative and descriptive and was challenged in the 1930s by the regional geography of Richard Hartshorne. Hartshorne called for systematic analysis of the elements that varied from place to place, a project taken up by the quantitative revolution. Cultural geography was sidelined by the positivist tendencies of this effort to make geography into a hard science although writers such as David Lowenthal continued to write about the more subjective, qualitative aspects of landscape.