Aristotle's biology is the theory of biology, set out by the ancient Greek philosopher Aristotle in the 4th century BC. It was the first attempt to systematically study and classify all forms of life. In his work, Aristotle proposed a hierarchical system of classification for living things, based on their physical characteristics. He also proposed that all living things had a purpose or telos, and that they could be divided into two main categories: plants and animals. He argued that plants were capable of reproduction but lacked sensation and mobility, while animals had both sensation and mobility but lacked reproductive capacity. Aristotle also made significant contributions to the study of anatomy, physiology, embryology, ecology, and behavior.