It’s not even that we lack a way to define fish, it’s more that we lack a definition that isn’t arbitrary. One can define them as something like “vertebrates, except for all these ones that we don’t want to include”, but then there’s not really a clear reason to exclude all the amphibians and reptiles and mammals and such, other than that they don’t traditionally get called fish. Some of them even live in water, and a handful of fish can leave the water to a limited extent, so it isn’t even that.
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.
Just because we lack a definition, doesn’t mean something doesn’t exist.
It’s not even that we lack a way to define fish, it’s more that we lack a definition that isn’t arbitrary. One can define them as something like “vertebrates, except for all these ones that we don’t want to include”, but then there’s not really a clear reason to exclude all the amphibians and reptiles and mammals and such, other than that they don’t traditionally get called fish. Some of them even live in water, and a handful of fish can leave the water to a limited extent, so it isn’t even that.
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.