When is Life?

Abortion is one of the most heated and emotionally resonating issues of our time. Medical advancements have made it possible for women to end a pregnancy, prevent pregnancy as well as save the lives of their prematurely born children.

It all really boils down to when we believe life starts or how we define "life" itself. Some say viability but that has changed drastically in the last 20 years. Some say conception. Some say birth. Since this question has shaped our political system for decades now, I think it's about damned time we talk about it.

When do you think human life starts? Why? How do you define life?

(I do not mean how we should legally classify life, though if you want to answer that too, go ahead. )