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In 1943 Erwin Schroedinger held a series of talks discussing the question - What is life? He said that life created order from order (by genetics) and order from disorder (metabolism and growth). He also noted that for the laws of thermodynamics to be obeyed, organisms create order within themselves by creating greater disorder in the environment.
There are attributes that all life seems to share. All living things can self replicate, but this does not mean that all things, even in nature, which can self replicate are alive. Crystals can self replicate in a solution, but this does not make them alive, because they are unable to evolve mutations. Another example that seems contrary to this, is that not all members of a species need to be able to self replicate in order for a member of the species to be alive. For example, in a bee colony, only a few of the bees are capable of reproduction. The rest are workers. Many hybrid animals (such as mules) are sterile, but they are certainly alive as well. Also, most organisms are only able to reproduce during a certain period of their life cycle, but this does not mean they are not alive at other times of their lives.
All living things are chemical systems (as opposed to mechanical or electronic). Robots, or other mechanical systems can be programmed to create replicas of themselves, but because their constituent parts are larger than molecules, it is extremely improbable that such a system would develop independently.
The definition of life needs to be broad enough to include all of these examples of life, and exclude the examples that have similarities but are not alive: Life (at the species level) is a self-replicating chemical system capable of evolving in such a way that its offspring might be better suited for survival.
Another property of life that is interesting is called the anthropic principle. Living systems seem so complex, it may seem incredibly unlikely that they could have ever arisen. However, the anthropic principle states that living observers will always find themselves in a universe with the physical properties necessary for the existence of life. This doesn’t tell us anything about the probability though. We could be the only example of life, or there could be billions of others. Until we discover life elsewhere in the universe, we just won’t know.
