There is one key pro-life argument: at some point, we consider an entity human, and it is illegal to kill humans (generally speaking). The question is, when is a fetus/baby considered a human. You can argue that it's upon delivery, upon its ability to be self-sustaining outside the womb, upon its ability to survive with assistance outside the womb, when a certain level of brain function starts, when there's a heart beat, upon conception, etc. To say there's no pro-life argument to be made is ignorant, intentionally or otherwise. BTW, pro-choicer here.
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What is the "liberal's" solid definition of what defines a person? As a pro-choice liberal, I'd like to know what definition you think I am using.