What Does it Mean for Something to be Alive?

by | Mar 20, 2018

What exactly is life? A tree  is alive, but how about the wood from a tree used to build a table? Is fire alive? It takes in oxygen, can grow and reproduce. How about Artificial Intelligence, is there some point at which a machine could be considered alive? If a computer gains consciousness, but does not reproduce is it “alive?” What about “life” on other planets, could extraterrestrial life have very different characteristics? Could it be based on silicon instead of carbon or be an entirely un-imagined type of life? A big debate in biology is whether viruses are considered alive.


Are viruses alive?

The precise definition of life is elusive. Philosophers and scientists have spent thousands of years debating what exactly being alive means and we still don’t have a satisfying, all-encompassing answer. There are hundreds of current definitions of life.

As an example of what makes this so hard, here’s the definition of life NASA has adopted: “life is a self-sustaining chemical system capable to undergoing Darwinian evolution.” A good definition, but also with problems – what about mules? They are alive but are incapable of reproduction and thus don’t evolve. Notwithstanding some problems with the NASA definition, it seems to be pretty widely accepted.

Some definitions just list characteristics. The common attributes taught in high school biology list seven: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. But, a number of non-living things have these attributes, including some crystals called prions and some computer programs. Even fire arguably can pass the test of life under these criteria.

Other criteria that are generally accepted under most circumstances are:  (a) an ability to replicate, (b) an existence bounded by birth and death and (c) a degree of biochemical autonomy meaning that the organism carries on metabolic activities that produce the molecules and energy needed to sustain itself.

The unsatisfying answer is that there is no universal accepted definition of life. A chemist, biologist, astrobiologist and computer scientist might all have different definitions.


  1. I have a simpler classification for whether something is alive or not. To be alive, it must consume something that was alive. It cannot consume something that was never alive exclusively. It must consume life, and excrete nutrients that are produced or refined through metabolic processes. In essence, you have to eat a burger, to be a cow.

  2. I’m beginning to think the IFod has a life of its own.

  3. Existence bound by life and death works for me.

  4. Maybe the question should be, “What is the definition of intelligent life.” The problem with asking that question is that you and I might not fit with in the bounds of the somebody else’s definition. Ah the conundrum continues!!


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