This thought is taken from Peter Kreeft.
Ether a fetus is a person or it isn’t.
The fetus is a person and we know it = homicide.
The fetus is a person and we don’t know it = manslaughter; similar to shooting into a bush that moves because we think it might be a deer and it turns out it was a fellow hunter.
The fetus isn’t a person and we don’t know it = criminal negligence; abortion is just as irresponsible as it is in the previous case. You shot into the bush without knowing that there were no persons there. You were lucky; there weren’t. But you didn’t care; you didn’t take care; you were just as irresponsible. You cannot legally be charged with manslaughter, since no man was slaughtered, but you can and should be charged with criminal negligence.
The fetus isn’t a person and we know it = abortion a reasonable, permissible, and responsible choice. But note: What makes Case 4 permissible is not merely the fact that the fetus is not a person but also your knowledge that it is not, your overcoming of skepticism. So skepticism counts not for abortion but against it. Only if you are not a skeptic, only if you are a dogmatist, only if you are certain that there is no person in the fetus, no hunter in the bush, may you shoot.