On a sofa in the corner of the room, a cat is purring. It seems obvious that the cat is an example of life, whereas the sofa itself is not. But should we trust our intuition?
Consider this: Isaac Newton assumed a universal time flowing without external influence, and relative time measured by clocks—just as our perception tells us. Two centuries later, Albert Einstein dropped the concept of universal time, and instead introduced a concept of time measured only locally by clocks. Who before Einstein would have thought that time on the Sun, the Moon, and even on each of our watches runs at slightly different rates—that time is not a universal absolute? And yet today our cellphones must take this into account for a GPS to function.
Science has made amazing strides, uncovering a deep and often counterintuitive understanding of physical reality. We understand a lot about the atoms in the human body and the stars in the night sky: much more than we do about the individual human as an example of life. In fact, life scientists continue to debate the exact definition of life. It was Aristotle who first said that life is something that grows and reproduces. He was fascinated by the mule, a cross between a horse and donkey that is always sterile. But just because the mule was sterile, you couldn’t call it dead. The debate is endless: some say that life must metabolise, that is, take in compounds, turn them to energy, and release some waste. But do jet engines qualify? In short, there is no theory and therefore no measuring apparatus that can confirm or refute our assumption that the cat is alive and the sofa is not, nor even that you are alive as you read this.