`We are surrounded with things which we have not made and which have a life and structure different from our own: trees, flowers, grasses, rivers, hills, clouds. For centuries they have inspired us with curiosity and awe. They have been objects of delight. We have recreated them in our imaginations to reflect our moods. And we have come to think of them as contributing to an idea which we have called nature. Landscape painting marks the stages in our conception of nature. Its rise and development since the Middle Ages is part of a cycle in which the human spirit attempted once more to create a harmony with its environment.'You can read the 1949 edition of this book online on The Internet Archive. It also appears to be available to download from the Internet Archive. You can also now preview this book on Google Books - if you find the right link! Above is a screendump of the contents page.
I have to get a 'proper' copy of this book not least because I love his categories of how landscape art works