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Most of us probably do not need to think too hard to distinguish living things from the “non-living”. A human is alive; a rock is not. Easy!

Scientists and philosophers do not see things quite this clearly. They have spent millennia pondering what it is that makes something alive. Great minds from Aristotle to Carl Sagan have given it some thought – and they still have not come up with a definition that pleases everyone. In a very literal sense, we do not yet have a “meaning” for life.

If anything, the problem of defining life has become even more difficult over the last 100 years or so. Until the 19th Century one prevalent idea was that life is special thanks to the presence of an intangible soul or “vital spark”. This idea has now fallen out of favour in scientific circles. It has since been superseded by more scientific approaches. Nasa, for instance, has described life as “a self-sustaining chemical system capable of Darwinian evolution”.

But Nasa’s is just one of many attempts to pin down all life with a simple description. In fact, over 100 definitions of life have been proposed, with most focusing on a handful of key attributes such as replication and metabolism.

To make matters worse, different kinds of scientist have different ideas about what is truly necessary to define something as alive. While a chemist might say life boils down to certain molecules, a physicist might want to discuss thermodynamics…

A comparative survey of the definitions that currently exist concludes…

To properly define life, we might need to find some aliens.

The irony is that attempts to pin down a definition of life before we discover those aliens might actually make them more difficult to find. What a tragedy it would be if in the 2020s the new Mars rover trundles straight past a Martian, simply because it does not recognise it as being alive.

“The definition can actually hinder the search for novel life,” says [Carol] Cleland. “We need to get away from our current concept, so that we are open to discovering life as we don’t know it.”

It is surprisingly difficult to pin down the difference between living and non-living things: “There are over 100 definitions of ‘life’ and all are wrong.“

* Carl Sagan

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As we strive for beginner’s mind, we might send exploratory birthday greetings to John Theophilus Desaguliers; he was born on this date in 1683. A natural philosopher, clergyman, and engineer, he is best remembered as the experimental assistant to Isaac Newton, who went on to popularize Newton’s work in public lectures and publications. On the strength of that work, Desaguliers was elected to the Royal Society and ultimately became its curator.

In his own work he coined the terms conductor and insulator. He repeated and extended the work of Stephen Gray in electricity. He proposed a scheme for heating vessels such as salt-boilers by steam instead of fire. And he made inventions of his own (e.g., a planetarium), and material improvements to others’ machines, such as Thomas Savery’s steam engine (by adding a safety valve and using an internal water jet to condense the steam in the displacement chambers) and a ventilator at the House of Commons. 

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