Greek philosopher Aristotle created two classification systems to group living organisms based on several factors, including physical characteristics and perceived mental capacities. Aristotle organized living organisms physically on the basis of movement as well, which included walking, flying and swimming and in intellectual hierarchy through the Great Chain of Being, also called scala naturae. Aristotle introduced his systems of scientific classification in the 1600s, which classified organisms until the 1800s.
Aristotle was born in northern Greece in 384 B.C. and initially studied medicine. He produced a classification based on a theory that all living organisms share some commonalities and traits. Aristotle devised a classification system for animals, which included, in descending order, vertebrates, invertebrates, arthropods living on land and sea, animals with shells, without shells and plant-like animals. Aristotle excluded fungi and true plants from this system of classification, which otherwise accounted for life forms on Earth. Aristotle believed living beings possessed different levels of intellectual capacity, which determined order in his system of scala naturae. Aristotle placed plants, which lacked intelligence and reasoning, at the bottom of his classification table while animals, with greater ability to reason, came next. Humans, the most intelligent and mentally complex beings, formed the top of Aristotle's hierarchical system of classification.