Aristotle, known as "the Philosopher" by later thinkers, created a huge body of work that was virtually synonymous with philosophy for over 2000 years. His most well-known doctrines include the notions that morally virtuous people seek moderation in all things (the "mean" between extremes); that the soul is the essence or the characteristic activity of the living body; that happiness is found not in mere pleasure, but in fully developing the powers of the soul in pursuit of excellence throughout a lifetime; and that in the good life we engage in the right activities for their own sake. 

Aristotle organized and classified an immense amount of knowledge, much of it scientific theories developed with only the crudest observational tools. All knowledge is organized into the theoretical disciplines (physics, "first philosophy" [metaphysics], and math); practical disciplines (ethics and politics); and productive disciplines (engineering, medicine, etc.). He classified four types of change (generation / corruption, increase / decrease, alteration, and locomotion) and four types of causation (the material, the formal, the initiating force, and the goal). Aristotle formed a metaphysical theory of what is real ("substance"), and described the idea that all things have "potential" and "actual" characteristics. He created logic to distinguish correct from incorrect reasoning; and he reasoned that all motion ultimately is caused by an immutable perfection, an unmoved "primer mover" - which Aristotle called "God." 

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