On a windy day in Ypenburg, the Netherlands, you can sometimes see sculptures the size of buses scuttling across a sandy hill. Made mostly from intricately conjoined plastic tubes, wood and sails, the many-legged skeletons move so fluidly and autonomously that it’s tempting to think of them as alive. Their maker, the Dutch artist Theo Jansen, certainly does. “Since 1990, I have been occupied creating new forms of life,” he says on his website. He calls them Strandbeest. “Eventually I want to put these animals out in herds on the beaches, so they will live their own lives.”

Poetic, most would say, but Strandbeest are not alive. They are just machines — elaborate, beautiful ones, but inanimate contraptions nonetheless. A few months ago I would have agreed with this reasoning. But that was before I had a remarkable insight about the nature of life. Now, I would argue that Strandbeest are no more or less alive than animals, fungi and plants. In fact, nothing is truly alive.

What is life? Science cannot tell us. Since the time of Aristotle, philosophers and scientists have struggled and failed to produce a precise, universally accepted definition of life. To compensate, modern textbooks point to characteristics that supposedly distinguish the living from the inanimate, the most important of which are organization, growth, reproduction and evolution. But there are numerous exceptions: both living things that lack some of the ostensibly distinctive features of life and inanimate things that have properties of the living.

Crystals, for example, are highly organized; they grow; and they faithfully replicate their structures, but we do not think of them as alive. Similarly, certain computer programs known as “digital organisms” can reproduce, mate and evolve, but ushering such software through the gates to the kingdom of life makes many people uncomfortable. Conversely, some organisms — such as gummy bear-shaped microanimals called tardigrades and brine shrimp (whose eggs are sealed up in little packets like baker’s yeast under the brand name Sea Monkeys) — can enter a period of extreme dormancy during which they stop eating, growing and changing in any way for years at a time, yet are still regarded as living organisms.