Whether or not someone considers it a human life doesn't mean it either is or isn't one, and that's exactly what we can't come to a decision on.

From Roe v. Wade:

We need not resolve the difficult question of when life begins. When those trained in the respective disciplines of medicine, philosophy, and theology are unable to arrive at any consensus, the judiciary, in this point in the development of man's knowledge, is not in a position to speculate as to the answer.

Now I don't know whether Roe v. Wade was ruled correctly or incorrectly, but I do believe that their lack of resolution for the definition of a life is what keeps the argument going today.