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The only hard definition of species is individuals that can potentially produce viable offspring with one another. However regional varieties can be so greatly diverged in morphology and behaviour while still being able to produce fertile offspring (See polar and grizzly bears) that scientists don't bother using it.
Instead they narrow the definition of species to be "different enough" and call it a day. Speciation is a gradual and ongoing process, pointing to the specific boundary is like pointing to the boundary between geological eras.
In short I think the current framework is flawed but good enough to save the effort and confusion of everyone arguing about hammering out a lens to view life on Earth.
