Ah yes, I guess I neglected to mention my saga-specific presumption, copied from Sir pTerry. To paraphrase:
A Human is where the Falling Angel meets the Rising Ape.
Essentially Human nature/soul is a combination of the Divine spark and animal nature, in complimentary conjoining.
All Humans potentially have an animal side, though few can express their heartbeast.
If you have not expressed your inner heartbeast, you can compensate with a compatible Familiar.
Someone without a HeartBeast of Familiar still has a complete soul, just may have difficulty expressing it properly.
If that makes sense.