Theophrastus of Eresus (c. 371–c. 287 BCE) was a Greek philosopher and scientist of the early Hellenistic period. A native of Lesbos, he studied in Athens, first under Plato and then under Aristotle. He became Aristotle’s closest associate and succeeded him as head of the Peripatetic school, the Lyceum, leading it for about thirty-five years.

He was an extraordinarily prolific writer, producing hundreds of works on logic, ethics, metaphysics, and natural science. Only a few of these survive. His most famous works are two long botanical treatises, Enquiry into Plants and On the Causes of Plants, which systematically describe and classify hundreds of plant species, their physiology, and their uses. These works are considered the foundation of botany in the Western tradition. Another well-known surviving work is Characters, a series of short, sharp sketches of negative personality types, which influenced later satire.

Modern scholars see Theophrastus as crucial for preserving and critically developing Aristotle’s ideas, ensuring the survival of the Peripatetic school. His significance rests on his role as Aristotle’s successor and, above all, on his pioneering empirical study of the plant world, which remained authoritative for over a thousand years.