The only factual argument that matters is that ZEFs are inside someone else’s body while infants are not. The change from the first state to the second is significant for that reason. Abstract notions like level of development or chances of survival are grey areas; whether someone is inside another person or not is black and white. If this doesn’t make sense, imagine that you’re standing in a crowded bus, and the guy next to you has his hand in your pocket, or in his own.
The only factual argument that matters is that ZEFs are inside someone else’s body while infants are not. The change from the first state to the second is significant for that reason. Abstract notions like level of development or chances of survival are grey areas; whether someone is inside another person or not is black and white. If this doesn’t make sense, imagine that you’re standing in a crowded bus, and the guy next to you has his hand in your pocket, or in his own.