When it’s a grouping that we lack the definition for, then the group doesn’t really exist, even if it’s members do and we all gave a good idea of what are, for instance, fish. Basically the group ‘fish’ contains all the things you think are fish, which is problematic as someone else may have a different idea of which things belong in the group, and while that’s fine when talking coloquially, you can’t really use it when trying to discuss things in a rigerous fashion.
When it’s a grouping that we lack the definition for, then the group doesn’t really exist, even if it’s members do and we all gave a good idea of what are, for instance, fish. Basically the group ‘fish’ contains all the things you think are fish, which is problematic as someone else may have a different idea of which things belong in the group, and while that’s fine when talking coloquially, you can’t really use it when trying to discuss things in a rigerous fashion.