Without other examples, it is difficult to distinguish essential properties of life - properties that would be shared by any living system - from properties that may be incidental to life in principle, but which happen to be universal to life on Earth due solely to a combination of local historical accident and common genetic descent.
In order to derive general theories about life, we neeed an ensemble of instances to generalize over. Since it is quite unlikely that alien lifeforms will present themselves to us for study in the near future, our only option is to try to create alternative life-forms ourselves - Artificial Life - literally ``life made by Man rather than by Nature.''
Artificial Life (``AL'' or ``Alife'') is the name given to a new discipline that studies "natural" life by attempting to recreate biological phenomena from scratch within computers and other "artificial" media. Alife complements the traditional analytic approach of traditional biology with a synthetic approach in which, rather than studying biological phenomena by taking apart living organisms to see how they work, one attempts to put together systems that behave like living organisms.
The process of synthesis has been an extremely important tool in many disciplines. Synthetic chemistry - the ability to put together new chemical compounds not found in nature - has not only contributed enormously to our theoretical understanding of chemical phenomena, but has also allowed us to fabricate new materials and chemicals that are of great practical use for industry and technology.
Artificial life amounts to the practice of ``synthetic biology'' and, by analogy with synthetic chemistry, the attempt to recreate biological phenomena in alternative media will result in not only better theoretical understanding of the phenomena under study, but also in practical applications of biological principles in the technology of computer hardware and software, mobile robots, spacecraft, medicine, nanotechnology, industrial fabrication and assembly, and other vital engineering projects.
By extending the horizons of empirical research in biology beyond the territory currently circumscribed by life-as-we-know-it, the study of Artificial Life gives us access to the domain of life-as-it- could-be, and it is within this vastly larger domain that we must ground general theories of biology and in which we will discover practical and useful applications of biology in our engineering endeavors.