Here’s a compelling argument I heard recently, which sort of demonstrates how slippery definitions are – even definitions we think we have an handle on.
Goes something like this: What is alive? Are viruses alive? Most people would say yes. But are they? Viruses consume nothing and they leave no waste. They’re unable to reproduce on their own. They cannot survive without a host. Wouldn’t we normally consider all of those elements of the definition of life?
Now consider fire. Fire can propagate itself. It consumes, and produces waste. It takes in oxygen and expels carbon dioxide. Under those criteria, fire sounds every bit alive as your aunt Sally.
This argument by itself serves as both an interesting philosophical exercise, and a cautionary warning to NASA that maybe they ought to broaden their life-searching scope. In the broader sense it reminds us that language can be tricky, and there are fewer absolute truths than we’d like to believe.