It's not hard to form definitions, it's hard to form ones that people will agree upon because there are many different interests in play and many different values. And it's not really that difficult to know whether a proverbial 'clump of cells' is developing into what everyone normally agrees to be 'human' or not.

The hard part is in forming the laws and deciding whether they should value undeveloped members of the homo sapiens genus for their species, for being intelligent, their capacity to suffer, or other reasons, then balancing whatever rights are assigned against the rights of others.