Like his teacher Plato, Aristotle had tremendous impact on the development of natural philosophy. This lecture introduces Aristotle, his writings, and his ideas as a response to his predecessors, the Presocratics and Plato. We focus on Aristotle's views on the value of observation, the nature of change, the composition of matter, and what constitutes real knowledge. The characterization of Aristotle as a "biologist" helps to make sense of his worldview, contrasted with the modern worldview based instead on physics.