Greek philosophers such as Aristotle defined life by the presence of some form of soul, and the idea that there must be a single unique feature associated with life has always remained popular. In the 1800s the notion of a "life force" was discussed—and thought to be associated perhaps with chemical properties of protoplasm, and perhaps with electricity. The discovery from the mid-1800s to the mid-1900s of all sorts of elaborate chemical processes in living systems led biologists often to view life as defined by its ability to maintain fixed overall structure while achieving chemical functions such as metabolism. When the Second Law of Thermodynamics was formulated in the mid-1800s living systems were usually explicitly excluded (see page 1021), and by the 1930s physicists often considered local entropy decrease a defining feature of life. Among geneticists and soon mathematicians self-reproduction was usually viewed as the defining feature of life, and following the discovery of the structure of DNA in 1953 it came to be widely believed that the presence of self-replicating elements must be fundamental to life. But the recognition that just copying information is fairly easy led in the 1960s to definitions of life based on the large amounts of information encoded in its genetic material, and later to ones based on the apparent difficulty of deriving this information (see page 1069). And perhaps in part reacting to my discoveries about cellular automata it became popular in the 1980s to mention adaptation and essential interdependence of large numbers of different kinds of parts as further necessary characteristics of life. Yet in the end every single general definition that has been given both includes systems that are not normally considered alive, and excludes ones that are. (Self-reproduction, for example, suggests that flames are alive, but mules are not.)
One of the features that defines life on Earth is the presence of DNA, or at least RNA. But as one looks at smaller molecules they become less specific to living systems. It is sometimes thought significant that living systems perpetuate the use of only one chirality of molecules, but actually this can quite easily be achieved by various forms of non-chemical input without life.
The Viking spacecraft that landed on Mars in the 1970s tried specific tests for life on soil samples—essentially whether gases were generated when nutrients were added, whether this behavior changed if the samples were first heated, and whether molecules common in terrestrial life were present. The tests gave confusing results, presumably having to do not with life, but rather with details of martian soil chemistry