Peter Kreeft’s Argument Against Abortion:

There are only four possible situations: 

1. The fetus is a person and we know it. Abortion could not be acceptable.

2. The fetus is a person but we aren’t sure. Abortion could not be acceptable. 

3. The fetus is not a person but we aren’t sure. Abortion could not be acceptable because at best we would only be guessing. This would be like collapsing a skyscraper before you were sure all the people were out. At best it is irresponsible and only lucky if the fetus really is not a person. 

4. The fetus is not a person and we know it. This would be the only situation where abortion can be morally permissible. The problem is, no one can demonstrate that the fetus is not a person without just arbitrarily defining the fetus as a non-person. No one has ever proven the non-personhood of a fetus. Any benefit of the doubt must go to life.