Despite the abundance of life surrounding us, there is currently no generally accepted definition of life. In the 1970s, Carl Sagan, astronomer and exobiologist, attempted to define life through five categories of definitions. However, problems arise in the form of counter-examples of objects we would not consider “alive,” but fulfill the requirements of the definition.
Through the physiological definition, life is defined as a system capable of performing a number of functions including eating, metabolizing, excreting, moving, growing, reproducing, breathing, and responding to stimuli. However, cars could be defined to “eat,” “metabolize,” “excrete,” “move,” and “respond to stimuli” as well. Does that make cars alive? Fire also serves as a sufficient counter-example. There are even certain types of bacteria that don’t breathe at all but serve their days by altering the oxidation state of sulfur. But we would consider bacteria more alive than cars, wouldn’t we?
The metabolic definition states that life uses energy to perform functions within its boundaries, by exchanging material with its surroundings without altering its general properties. Cars can also be considered “alive” through this definition as well. There are spores and seeds that are capable of being dormant and without any metabolic activity for hundreds of years, and can revive perfectly under better conditions. Does the definition of these spores and seeds change throughout their existence? Are they alive while they are performing metabolic functions but not alive while dormant?
The biochemical definition defines life as having a distinctive chemistry and contains coded hereditary information (ex. DNA) that is passed to the next generation. According to this definition, mules (offspring of a horse and a donkey) are not “alive” because they cannot reproduce. Viruses also have distinctive chemistry but cannot reproduce on their own. However, it is an ongoing debate as to whether viruses are alive or not.