For scientists eying distant planets and solar systems for signs of alien activity, University of Colorado at Boulder Professor Carol Cleland suggests the first order of business is to keep an open mind. It may be a mistake to try to define life, given such definitions are based on a single example -- life on Earth, said Cleland, a philosophy professor and fellow at the NASA-funded CU-Boulder Center for Astrobiology. The best strategy is probably to develop a “general theory of living systems,” she said.