Defining “life” is difficult. I’ve heard it said that while we can’t really define life that “we know it when we see it.” Viruses are proof that this truism is not true. With viruses we see it, and then argue about it.
A good definition should be categorically exhaustive and mutually exclusive. By “categorically exhaustive” we mean the definition should include everything we regard as included in the category, for example the definition of “door” should include all doors. By “mutually exclusive” we mean that nothing we do not regard as included in the category are included by the definition. For example, the definition of a door should not allow the inclusion of a window.
If we define life as “DNA capable of replicating”, do viruses qualify?