What is life? What is a soul? These are some of the questions Greek philosopher Aristotle asked. And we're still asking them today.

Aristotle also could be considered the world's first natural historian. Acclaimed biologist Armand Marie Leroi, in his new book, The Lagoon: How Aristotle Invented Science, follows in Aristotle's footsteps to the Greek island of Lesbos, where Aristotle—the man Plato called "the foal"—did much of his hands-on research.

Speaking from his home in London, the author describes how a chance encounter in an Athens bookshop led him on a journey of discovery, why it's important to get your hands dirty if you want to understand the world, and why, among other things, Aristotle thought blondes have the best orgasms.

Your book begins with a childhood memory. Take us back there.

When I was 11 or so, I became interested in seashells. I was living in South Africa at the time, and I discovered in the garage an old flight bag full of seashells, which my parents had picked up. It became a nucleus of a shell collection. There was one shell in particular, which my parents had bought on their honeymoon in the Mediterranean. It was a trumpet shell, a very beautiful thing, and I pinched that too [Laughs]. I was going to be the world's most famous malacologist: a snail biologist. I never became a malacologist. But I did become a biologist. And that's where it began.

Thirty years later, long after I'd put away my shells, I walked into a bookshop in Athens and discovered a book called Historia Animalium, by Aristotle. I wasn't at all interested in Aristotle or philosophy. I was a biologist. But the title attracted me. So I took it down and started reading. There was Aristotle, this guy who had died 2,300 years ago, describing my shell. And I understood exactly what he was saying.

You imagine at one point Aristotle "breakfasting on figs and honey." What do we know about Aristotle, the man?

Remarkably little. Aristotle leaves behind several thousand pages of work, but none of it autobiographical. What we have in terms of autobiography is hearsay—stories related by later philosophers, typically written several centuries after his death. You can try to winnow out some truth from them but it's very hard because they often come from competing philosophical schools, and so they're notoriously unreliable.

We do have a description of him, and it's not a very attractive one. He's got bandy legs and small eyes. He also seems to have been a bit of a dandy, who fussed around with his hair. But once again it's not clear whether this is an accurate description—or a hostile one. It's certainly written by someone several centuries after Aristotle died, in 322 B.C.

The most immediate document we have is a reproduction of his will. Here we have a sense of Aristotle, the man. He speaks, for example, of how he'd like to be buried next to Pythias, his first wife, because that's what she would have wanted. She was young when Aristotle married her—he was around the age of 37. So we have a sense that he loved her very much.

View Images Both Plato and Aristotle taught at an academy in Piraeus, the port area of Athens. Photograph by Leemage/UIG via Getty Images

You give a wonderful description of the academy in ancient Athens where Aristotle and Plato lectured. Give us a virtual tour.

The academy was in Piraeus, the port area of Athens. There's not much left of it now, just some rocks and a dusty field with some trees. In its day it was something like a philosophical club. Not a university but a group of friends, with some sort of hierarchy. Clearly, Plato is at the top. There are other, young teachers sitting about, arguing and quarreling, but in a friendly spirit.

What makes Plato's academy special is that although it seems to be one of several different schools in Athens, Plato didn't charge for admission. He's reasonably well off, and it's clear that the academy is infused with his purpose, which is the pursuit of philosophical truth. The schools that the Sophists ran were all about teaching young men how to speak well, and get on in life. In modern parlance, you went to Plato for a Ph.D. and to the Sophists for an MBA.

There was lots of competition between Plato and Aristotle, wasn't there?

There's quite a bit of competition. Aristotle is 17 years old when he comes to the academy. It's clear he must be very bright because he quickly becomes known as "the reader." Sometimes he's also called the brains of the school.

Later on, as Aristotle gets older and more confident, Plato refers to him as the foal, by which he means Aristotle kicks his mother, as a foal kicks his dam. In other words, he's an ungrateful recipient of the good things his forebears were giving him.

Disputes were inevitable. We have two of the greatest thinkers of all time right next to each other, and they will turn out to have very different views of the world, even though their views are also deeply intertwined, as pupil and teacher must be. When Plato dies, Aristotle is 37, and right around then he leaves the academy and strikes out on his own.

This is a scholarly work. It's also a travelogue and a memoir. Tell us about the lagoon.

When we imagine a typical Greek island, like in the Cyclades, we think of a bare rock with some whitewashed houses on it. Lesbos is not like that. It's a large, ramshackle, agricultural island. Half of it is quite dry, half of it is wonderfully forested, with rivers and meadows and wildflowers. It's also a resting place for millions of migratory birds that go between Africa and the north every year. So there's lots of nature.

Bisecting this beautiful island is a lagoon 13 miles long, 6 miles wide, and 30 feet deep. It turns out to be one of the richest bodies of water in the Aegean, the nursery ground for the local fish. There are sardines and cuttlefish—all the creatures Aristotle describes. We have no definitive proof, but it seems likely that his descriptions of the lagoon are the oldest descriptions of any natural place in the world.

If you go down to the lagoon today, you'll see a series of little fishing villages. Some of them were bigger in Aristotle's time. They use little boats called trehantiri. In Aristotle's time they would have had sails instead of motors. But the designs aren't terribly different, and the techniques by which they fished aren't very different. For example, every year the fishermen throw bundles of branches into the water for the cuttlefish to lay their eggs on to ensure a good crop the following year. And this is exactly what Aristotle described 23 centuries ago!

View Images Aristotle did much of his research about the natural world on the Greek island of Lesbos. Photograph by Mary Mills Patrick

Aristotle described about 500 creatures in his immediate environment. He also described animals he could not have seen, like the elephant and the lion. How did he get his information?

Aristotle never left the Aegean, but he describes a lot of exotica, like African and Asian animals. How did he know all this? There's a wonderful story by Pliny—that Alexander the Great ordered all the commanders in his army, throughout Asia Minor, to send Aristotle specimens of the creatures they encountered. Why would he do this? Because Aristotle taught the young Macedon prince who would go on to become Alexander the Great. According to Pliny, this is the homage of the young general to his much-loved teacher.

It's a wonderful story. And I think it's almost certainly a myth, because so much of what Aristotle says about exotic creatures is dodgy. When he talks about Greek animals—a cuttlefish, for example—he speaks about them in such detail and with such accuracy that you get a sense that he actually saw the thing, and even cut it up. He did a lot of dissection.

But as soon as he starts talking about exotica, it all becomes much vaguer. He knows a lot about elephants, but he thinks they're semiaquatic. That's not entirely wrong. Elephants do like mucking about in water. But he's unsure as to how bendy their legs are. That later gave rise to a myth that elephants couldn't bend their legs and slept upright.

As well as following in Aristotle's footsteps, you did lots of hands-on research, observing and even dissecting the creatures Aristotle studied. Tell us about the soul of a cuttlefish.

[Laughs] So what is the soul of the cuttlefish? The first thing that's remarkable about Aristotle is that you can take a cuttlefish from the lagoon and pretty much dissect it following his instructions. I did this myself. It's the only way to see what he's doing.

If I've made any contribution toward the study of Aristotle, it's in doing things like this. Philosophers love to study these things in the privacy of their studies or libraries. Many of them don't even bother to go to Greece let alone look at the cuttlefish Aristotle describes. They know their Aristotle, they know their Greek—bless them. But you have to wonder: Don't you think you should go out and get your hands a bit dirty, in order to see what he was actually looking at?

You're an evolutionary biologist. Aristotle also explored evolution. To what extent did he prefigure Darwin?

He prefigures Darwin in many different ways, except the one way that really matters. That is to say, he doesn't get to evolution. I know that sounds paradoxical. Like Darwin, he looks at the world like an architect or an engineer. He looks at biological creatures and asks: What are they for, and what are their parts for? For example, he analyzes the diversity of birds—their beaks, their legs, and so on, in terms of their function and their fitness to the environment. What we would now call adaptations.

But Aristotle is not an evolutionist. He's not a creationist, either. He's something very much stranger: an eternalist. According to Aristotle, the world is a perfectly engineered system, in which creatures are fitted to their environment and fitted to each other. How did the system originate? Aristotle says that's the wrong question. It's always been there. It's a very strange idea for us today, because our entire worldview is conditioned by the opposition between creationism and evolutionism.

View Images Aristotle described hundreds of animals, including those he could not have seen—such as the elephant and lion. Illustration by Corbis

He also wrote extensively about sex, both animal and human, including the female orgasm. What was his view?

He doesn't have a technical word for orgasm. But it's very clear what he's talking about. And he's very clear that women have pleasure in intercourse. Extreme pleasure. Pleasure equivalent to that of a man's. Blondes get particularly aroused, he says. And Aristotle thinks this is a good thing.

The only question he's got is whether [intercourse] is absolutely necessary in order to conceive. He has this elaborate theory about female fluids and reproduction. He spent a lot of time trying to separate out female fluids: menstrual fluids from vaginal lubrication from urine from female ejaculation. It all gets terribly complicated.

You write that Aristotle confronted the biggest of all questions: What is life? How did he answer?

Life is a thing that has a soul. All living things have souls. Dead things do not. Inanimate things do not. That sounds mystical because we're conditioned to think about the soul, in the Judeo-Christian tradition, as some immaterial entity that survives us when we die. Aristotle's predecessors, like Plato, had a conception of the soul not unlike that. They thought that the soul is something unique to humans, and immortal.

Aristotle says this is nonsense. The soul should be conceived of as the thing that animates all living things, all plants and animals. When a creature dies, its soul dies too. The soul is part of the system that maintains the creature's life.

That concept runs throughout the history of biology but in many ways is only now receiving its full fruition. Twentieth-century science was all about taking creatures apart, reducing them down to their cells, molecules, and genes. Twenty-first century science is all about putting them back together again.

That is Aristotle's great insight. He says it's fine to understand things in parts, and he devotes a lot of time doing so ... Sometimes he's right. Sometimes he's wrong. But his big idea is that you have to put it all together again.

That's why I say he's the very first systems biologist—and why finally, in the 21st century, when we ourselves have begun to put systems together, we can see more clearly what he was up to than ever before.

You end the book back at the lagoon with a declaration of love and admiration for Aristotle. What do you revere most?