Swedish naturalist Carl Linnaeus identified the three kingdoms of nature (stones, plants and animals) in his Systema Naturae (1735). Each kingdom was subdivided into classes, orders, genera, species, and varieties. Linnaeus identified six classes of animals, including mammalia, a term he coined that referred to animals that suckled. For Linnaeus, the naming and classifying of living beings was the key to all knowledge, especially the knowledge of God's plan for the world.