This book challenges several widespread views concerning Aristotle's methods and practices of scientific and philosophical research. Taking central topics in psychology, zoology, astronomy and politics, Professor Lloyd explores generally unrecognized tensions between Aristotle's deeply held a priori convictions and his remarkable empirical honesty in the face of complexities in the data or perceived difficult or exceptional cases. The picture that emerges of Aristotle's actual engagement in scientific research and of his own reflections on that research is substantially more complex than is usually allowed.