Cross Breeds - Why are cats and dogs not considered a species under the biological species concept?

Why are cats and dogs not considered a species under the biological species concept?

Do you mean that dogs and cats aren't the same species? Or that cats as a whole or dogs as a whole are species? Cats and dogs are the same species because they can't interbreed. But cats and dogs are independent species, although their nomenclature is sometimes argued (a cat is sometimes classified as Felis silvestris, Felis catus or even Felis silvestris catus and the dog is sometimes Canis lupis, Canis familiaris or even Canis lupus familiaris).