The interests of Theophrastus were wide ranging, extending from biology and physics to ethics and metaphysics. His two surviving botanical works, Enquiry into Plants (Historia Plantarum) and On the Causes of Plants, were an important influence on Renaissance science. There are also surviving works On Moral Characters, On Sense Perception, On Stones, and fragments on Physics and Metaphysics. In philosophy, he studied grammar and language and continued Aristotle's work on logic. He also regarded space as the mere arrangement and position of bodies, time as an accident of motion, and motion as a necessary consequence of all activity. In ethics, he regarded happiness as depending on external influences as well as on virtue.
Most of the biographical information we have of Theophrastus was provided by Diogenes Laërtius' Lives and Opinions of Eminent Philosophers, written more than four hundred years after Theophrastus' time. He was a native of Eresos in Lesbos. His given name was Tyrtamus (Τύρταμος), but he later became known by the nickname "Theophrastus," given to him, it is said, by Aristotle to indicate the grace of his conversation (from Ancient Greek Θεός "god" and φράζειν "to phrase", i.e. divine expression).
After receiving instruction in philosophy in Lesbos from one Alcippus, he moved to Athens, where he may have studied under Plato. He became friends with Aristotle, and when Plato died (348/7 BC) Theophrastus may have joined Aristotle in his self-imposed exile from Athens. When Aristotle moved to Mytilene on Lesbos in 345/4, it is very likely that he did so at the urging of Theophrastus. It seems that it was on Lesbos that Aristotle and Theophrastus began their research into natural science, with Aristotle studying animals and Theophrastus studying plants. Theophrastus probably accompanied Aristotle to Macedonia when Aristotle was appointed tutor to Alexander the Great in 343/2. Around 335 BC, Theophrastus moved with Aristotle to Athens, where Aristotle began teaching in the Lyceum. When, after the death of Alexander, anti-Macedonian feeling forced Aristotle to leave Athens, Theophrastus remained behind as head (scholarch) of the Peripatetic school, a position he continued to hold after Aristotle's death in 322/1.
Aristotle in his will made him guardian of his children, including Nicomachus with whom he was close. Aristotle likewise bequeathed to him his library and the originals of his works, and designated him as his successor at the Lyceum. Eudemus of Rhodes also had some claims to this position, and Aristoxenus is said to have resented Aristotle's choice.
Theophrastus presided over the Peripatetic school for thirty-five years, and died at the age of eighty-five according to Diogenes. He is said to have remarked "we die just when we are beginning to live".