Detecting Life

From DefinitionOfLife

The question of "What is life" is important for space exploration. One stringent test in actual use for life-detecting spacecraft (e.g. missions to Mars) is to point them at the Earth and see if they can detect life there.

That has nothing to do with "what is life" and everything to do with "how do you detect life" which is a completely different question. Conflating these two radically different concepts is erroneous. At best, the above comment is completely irrelevant.

Huh ? What's the point of defining some category, if there's no way to detect whether something falls into that category ? -- DavidCary

Detecting life isn't the same problem as ascertaining whether or not a given entity is alive. Endolithic bacteria came as a surprise, even though we knew what they were when we noticed them. Many parasites fall into the same category - it's not always easy to tell when someone is infected, despite knowing what an infection entails. And the same applies to many other relative inconspicuous organisms.