Categorization is you.

Mammal is a category. A category created by humans. You may scoff at that statement and respond with something about evolution and common ancestors and yes, absolutely, those are some of the metrics by which humans decide what belongs in the Mammal category. But humans decided there needed to be a category and humans decided by what metrics to separate animals. And when there is any ambiguity humans may (maybe even arbitrarily sometimes) have to make a decision about what belongs where. 

A more clear-cut example is planets. It used to be said that there were nine planets in our solar system. Now there are eight. This was a decision that was made (a vote to be more specific) because more "planets" were discovered that were in many ways similar to Pluto. For whatever reason, the decision was made to not include these new things as planets and instead create a new definition of planets that excluded them and, in the process, Pluto. 

From a less scientific point of view these kinds of categorizations happen in our day to day lives as well. Water with added minerals is still usually categorized as water. Water with a lot of added minerals may be considered a slurry. A bit of flavoring in water is flavored water. A lot of flavoring in water is soda or lemonade or tea. We can all agree on this in theory but in practice the exact lines where these categories end is going to vary based on the tastes of the individual. 

I've heard so many arguments about what fits a category "it's not a game if there's no goal to it" "that's not real music, it's just noise" "Christianity isn't a religion, it's a relationship" I could probably go on forever because categories influence all parts of our lives. 

I don't think I've built up to my point very well, but words are eluding me and it's getting late, so I'll just say it and hope the point lands. I think the way we categorize things and especially when we argue about what "objectively" fits in an entirely constructed category tends to say more about us both as individuals and as a species than it does about the universe we are trying to make observations about.