What you are questioning is actually a hotly debated topic. Different groups have very different opinions as to such definitions of words. Your particular question is very apropos for the abortion debate going on in America today. The debate literally boils down to a disagreement as to what is "human."
One solution is to have smoother laws without sharp edges between inanimate/wild-animal/pet/human. However, writing such laws in a way that people find acceptable turns out to be a remarkable challenge, so we live with the fact that disagreement over the edges will be hotly debated in exchange for "something that doesn't spectacularly fail in every way."
If you are interested in a debate on what it means to "set rules to determine..." I highly recommend reading about Gödel's incompleteness theorems. Many intuitively tempting phrasings for such rules fall into the sorts of First Order Logic structures Gödel was looking at. He found such structures had some particularly nasty side effects which have profound implications on your question of "should we set rules..."
These implications are so profound that I would almost dare to answer your question with "no, we should not set such rules" simply because it is so hard for a layman (with less than a college math degree) to write rules which don't fall victim to the incompleteness theorems. The only reason I would not answer "no" is because I think there are some partial rules which are beneficial, but sidestep Gödel's frustrating issues, and I think it can be good for society to seek those partial rules out. They should just understand that any attempt to make those partial rules into complete rules is a very tricky business indeed.