There are many definitions of life, but none is foolproof. Some biologists define life as the ability to reproduce. Others do not. For instance, a mule, which is the offspring of a female horse and a male donkey, is considered alive by most biologists. Ultimately, scientists decided that reproduction is an essential property of life, and that a lone rabbit does not meet this requirement.
Other definitions include the active resistance to external perturbations, the ability to change, and the capacity for evolution. These features, along with others, give living things many facets. Although the two definitions differ only in a slight amount, they refer to similar processes. For example, life must have a biological nervous system to be classified as living. A physicist may refer to something as living if it has a biological nervous system.
These core properties of life include responsiveness, growth, metabolism, energy transformation, reproduction, and evolution. In fact, life is more accurately cast as a process than as a noun. Individuals fall into different taxa and are comprised of cells capable of metabolic processes. Individuals are composed of a variety of characteristics, but the common characteristics of all living things give them the ability to evolve. A biological definition of life can be a good starting point for defining living things.
The genetic definition of life includes the concept of complexity. It cannot achieve this level of complexity without natural selection. However, it may be possible to build a machine that reproduces itself using preformed building blocks. Then, it can also reproduce instructions and would satisfy the genetic definition of life. The question remains whether these machines would evolve to a more complex form. A similar question arises with human-made artificial intelligence. There are many other ways to define life.
