NASA officials announced March 12 that ancient Mars could have supported primitive life. But this begs the question: What exactly constitutes life?
Merriam-Webster.com defines life as "an organismic state characterized by capacity for metabolism, growth, reaction to stimuli and reproduction." But there's no single satisfactory definition of life.
"I think it is a mistake to try to define life, because we have only one example of life, familiar life on Earth, and we have reason to believe that this example may be unrepresentative of life in general," Carol Cleland, a philosopher of science at the University of Colorado, Boulder, told LiveScience in an email.
Defining life Aristotle made the first attempt at a definition, describing life as something that grows, maintains itself and reproduces. But this definition would exclude mules, which are sterile, while including things like fire.
Calling life something that has a metabolism, the ability to take in energy to grow or move and excrete waste, is no good either; cars do this, for example.