artificial life
Chapter 1: Exposition
1.2 Defining Life
The definition of life is something that scholars, philosophers, and scientists have struggled with throughout history. "I think, therefore, I am" is easily the most famous thing that Rene Descartes ever said. If accepted it certainly allows humans to consider themselves alive, but beyond our species it still does not provide a clear definition of what it means to be alive. A bacterium certainly does not do anything that remotely resembles what we would consider thinking, but most would agree that a bacterium is a form of life.
Aristotle, the great philosopher of antiquity, actually attempted to define life. He believed that what distinguished forms of life from non-living objects was the possession of a soul. He went on to say that humans had the highest form of a soul and that animals and plants also had souls, but of lesser magnitude [Levy, 1992].
Aristotle’s definition held for centuries with little argument against it. Even those that might question his definition held to the belief that life could only originate from life and that something intangible is imbued upon an organism before it can be considered alive. However, the possession of a soul is too abstract a notion to use as a basis for determining whether something is alive. Life is an intangible element and any definition will always be partially determined by the philosophy of those giving that definition, but a much more concrete definition can be formulated.
Since the days of Aristotle it was observed that all living things possessed certain properties. They all consumed some sort of nourishment for energy, they all grew, they all reproduced and they all died. After Darwin published his observations on natural selection, evolution also became one of the defining characteristics of life. The discovery of DNA finally identified a property of all living organisms.
Between the postulation of evolution by Darwin and the discovery of DNA by Watson and Crick biology has been turned on its head. No longer is life defined in Aristotle’s terms. Life can be closely analyzed and compared. DNA holds all the information that makes us human, yet the same mechanism gives even the simplest organisms their form as well. Married with evolution, the mechanics of DNA alone are what give us the diversity of life on this planet.
When it comes right down to it, DNA is little more than information. We do not fully understand the mechanism that interprets and implements that information, how it can tell a dog from a daisy, but it seems clear that the key to defining life lies within DNA. The foundation of life is in the copying and interpretation of this information [Levy, 1992].