numsgil: Flame is just a chemical reaction that emits light, so I'm not sure that it even fits here. It's more of a process than an object in itself, but maybe life could arise as a form of organization within an energy flow... Maybe I should remove the word structure from the definition though. That might solve this issue.
prsn828: I don't think a tree would be non living. A tree has the ability to change in behavior between generations because of DNA mutations. Thus, it fits my definition.
There are definitely many complex things to take into account. As far as I've noticed, we tend to either end up calling everything alive or make a definition that doesn't allow for unknown forms of life.
Here's the wikipedia "conventional" definition:
1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.
2. Organization: Being structurally composed of one or more cells, which are the basic units of life.
3. Metabolism: Consumption of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
4. Growth: Maintenance of a higher rate of synthesis than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter. The particular species begins to multiply and expand as the evolution continues to flourish.
5. Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity as well as the composition of metabolized substances, and external factors present.
6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of higher animals. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun (phototropism) and chemotaxis.
7. Reproduction: The ability to produce new organisms. Reproduction can be the division of one cell to form two new cells. Usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.
By this definition I think some types of alife can be considered alive. Darwinbots and Evolve 4.0 seem to fit well enough, if any alife does.