"Felines" as a group exist only as a cognitive shortcut. Looking at the genetic and evolutionary data, we find a continuum where felines and other families bleed together in a very fine gradient. We just happened to draw boundaries in the interest of productive discussion. We will never find "the essence of feline nature" because there is no such concrete thing as a "feline" which would have such.

For a human example: at what point do we stop being conscious when falling asleep?