In 345 B.C.E., two men took a trip that changed the way we make sense of the natural world. Their names were Theophrastus and Aristotle, and they were staying on Lesbos, the Greek island where tens of thousands of Syrian refugees have recently landed.

Theophrastus and Aristotle were two of the greatest thinkers in ancient Greece. They set out to bring order to nature by doing something very unusual for the time: they examined living things and got their hands dirty. They turned away from Plato’s idealism and looked at the real world. Both Aristotle and Theophrastus believed that the study of nature was as important as metaphysics, politics, or mathematics. Nothing was too small or insignificant. “There is something awesome in all natural things.” Aristotle said, “inherent in each of them there is something natural and beautiful.”

Aristotle is the more famous of the two men, but Theophrastus deserves equal bidding in any history of naturalism. Born around 372 B.C.E. in Eresos, a town on the southwestern coast of Lesbos, Theophrastus was 13 years younger than Aristotle. According to Diogenes Laërtius—a biographer who wrote his Eminent Philosophers more than 400 years afterwards but who is the main source for what we know about Theophrastus’ life—Theophrastus was one of Aristotle’s pupils at Plato’s Academy. For many years they worked closely together until Aristotle’s death in 322 B.C.E. when Theophrastus became his successor at the Lyceum school in Athens and inherited his magnificent library.

Many historians neatly divide them up with Aristotle doing zoology and Theophrastus doing botany, but that’s not quite true—both wrote about plants and animals, but those books have not survived. Chance, or the vagaries of history, have handed us Aristotle’s Historia animalium (Enquiries into Animals) and Theophrastus Historia Plantarum (Enquiry into Plants).

We can imagine them on a warm summer’s day walking through the forests of Lesbos, collecting plants or rubbing soil between their fingers. With 1,400 species, Lesbos’ flora is manifold and diverse because of the island’s close proximity to Asia Minor. Its eastern side is lush and fertile—oaks grow on the mountainous slopes and further up, they would have found sweet chestnut and pines. In the valleys were olive groves and Asian rhododendrons. Cutting through the middle of the island is an inland sea—often called a lagoon—which is rich in nutrients and marine life. Oysters, algae, breams, bass, sea urchins, crabs, and many bird species offered rich pickings for Aristotle and Theophrastus. There must have been many days when they rose early to meet the fishermen who unloaded their haul in order to select, say, a cuttlefish or a John Dory for their investigations. Imagine them sitting at a table in their lodgings picking apart leaves, examining the tendrils of a vine, or dissecting a snail to describe its stomach. Theophrastus and Aristotle were interested in everything, and enquiring endlessly. How are these similar? What are the differences? What’s inside? How do they reproduce?

Only two of Theophrastus’ botanical books have survived—Historia Plantarum and De Causis Plantarum—but they mark the beginning of what we understand as botany today because they are based on systematic empirical methods. Theophrastus used his own observations but also collected information from other regions—from first hand accounts, manuscripts and books. Based on reports written by Alexander the Great’s soldiers after their military campaign in India, for example, Theophrastus wrote the first accounts of cotton, pepper, cinnamon, frankincense, and the Banyan tree. He knew around 500 plants and described them in the greatest detail. He thought about ways to classify them. What kind of categories would make sense? A division between cultivated and wild plants? Or, between aquatic and land–based, as Aristotle had divided the animal kingdom? Did similar shapes of leaves reveal relationships between different plants? Theophrastus thought of analogies and distinctions. He asked: What is the best way to define a plant? And like a true Aristotelian: What is its essential nature? A “plant is a thing various and manifold, and so it is difficult to describe in general terms,” he said. And when he couldn’t find answers, he noted, “this then is matter of enquiry.” He finally settled on dividing the world of plants into four groups by their habit of growth: trees, shrubs, sub–shrubs and herbs.

Theophrastus tried to lift the veil of superstition to understand the natural world. He railed against those who advised to be guided by the moon when it came to practical garden matters: “One should not in fact be governed by the celestial conditions and revolution rather than by the trees and slips and seeds” and he dismissed other myths as having been revealed as “sheer fable.”

Among ancient naturalists, Theophrastus was uniquely sensitive to the relationship between plants and their environment. Habitat was so important, he wrote, because plants were bound to it. They were “not free from it like animals.” He explained that a plant thrives best in a “favorable place,” or as ecologist would say today, in its “niche.” He studied soil, moisture, temperature, wind, and exposure. He discussed the adaption of plants to a particular environment and even wrote about “mutation according to place.”

Theophrastus was interested in the effect plants had on each other and noted that olives, myrtles and pines thrived when growing together but that the almond tree was a “bad neighbor.” He saw how trees grew tall and more upright when they stood close together. He noticed that jays buried acorns, helping them to germinate, and that mistletoe “lives by taking the food that belongs to the tree’.”

Aristotle saw similar relationships between animals and their environment but believed that plants and animals had been created “specifically for the sake of man.” Theophrastus, on the other hand, insisted that plants had their own purpose and were not made to serve humans, to feed us, or to provide building materials. In the twentieth century, some historians and botanists began to call him the ‘father of plant ecology’.

Laërtius wrote that Theophrastus devoted “his whole leisure to learning” and that he was “a man of extraordinary acuteness.” He certainly must have been busy because he wrote almost 300 books and treatises with subjects ranging from politics, morals and law to others on animals voices, on love, history, and astronomy. “These works contain in all two hundred and thirty–two thousand and eight lines,” Laërtius reports rather precisely, but few have survived.

After Theophrastus succeeded his old friend and collaborator Aristotle as the head of the Lyceum, he taught there for more than three decades. Laërtius says 2,000 pupils attended his lectures. When Theophrastus died at 85, the whole of Athens mourned him, according to Laërtius. Nature and gardens were places of learning, Theophrastus believed, and bequeathed his garden and promenade to his friends “to hold a school in them and to devote themselves to the study of philosophy … and to use them in common as if they were sacred ground.”