The definition of life has long been a challenge for scientists and philosophers, with many varied definitions put forward.[16][17][18] This is partially because life is a process, not a substance.[19][20][21] This is complicated by a lack of knowledge of the characteristics of living entities, if any, that may have developed outside of Earth.[22][23] Philosophical definitions of life have also been put forward, with similar difficulties on how to distinguish living things from the non-living.[24] Legal definitions of life have also been described and debated, though these generally focus on the decision to declare a human dead, and the legal ramifications of this decision.[25] As many as 123 definitions of life have been compiled.[26] One definition seems to be favored by NASA: “a self-sustaining chemical system capable of Darwinian evolution.”[27][28]

The lack of knowledge of the characteristics of living entities does not prevent us from assuming 1 characteristic for our basis. The process is aware. The process does respond. Any entity responds but what we mean here is the process responds. If you push a rock it moves. If you expose metal to oxygen it rusts. This is the entity not the process. The process is missing. The life is missing. When we look for life, we look for this additional response. It’s presence indicates life in some form. The problem is not seeing it does not mean absence of life, only the absence of a sighting.