Those who like their categories cut and dried may wonder whether viruses are alive or not. Wise biologists do not struggle too much with such questions. ...
The problem with categorical thinking in biology is that evolution does not work like that. It actually works by whatever works working.
Humans, by instinct, like to put things in neat categories. Our schooling reinforces that; students in school are taught all kinds of classification rules. This can be useful, but you also need to realize that reality can be very messy.
It is also important to realize the danger if categorical thinking when it comes to dealing with people.