I will be blogging the great works of Rabbi Chaim of Brisk.
This is on page 95 of the black book version of his novelties on the Rambam.
INITIAL BACKGROUND (there will be more background filled in later, but this is enough for now):
PRINCIPLE 1 - A jew is allowed to save the life of another jew, even if one must violate a commandment of the Torah to do so. For instance, if it is the Sabbath, and a man is trapped under a building and may die, you are allowed to use machinery and all manner of tools to save him, even though such action violates the Sabbath.
PRINCIPLE 2 - You do not set aside one life for another because no one human being is more valuable than his fellow, i.e., who is to say that your blood is redder than someone else.
PRINCIPLE 3 - If one sees a pursuer chasing a pursued, one is allowed to defend the pursued by applying whatever force is necessary to the pursuer to save the pursued, e.g., if the pursuer has a gun to the pursued's head and the only way to save the pursued is to push the pursuer of the roof and kill him, that is permissible. This is based on the Torah verse "one who spills the blood of man, by man his blood shall be spilled" - in essence, by attempting to murder the pursued, the pursuer has forfeited his life and put it in man's hands to end his life.
PRINCIPLE 3 is key because without it, there would be no way to save the life of the pursued at the expense of the pursuer under PRINCIPLE 1 (Save a Life) without violating PRINCIPLE 2 (No Set Aside of a Life). PRINCIPLE 3 (the Pursuer) essentially sets aside the life of the pursuer so that PRINCIPLE 2 is overriden (at least partially, as we will see).
Chapter 1, paragraph 9 of Rambam's laws of murder:
"The precept that you are not to have mercy on the soul of a pursuer is a commandment (that you may not violate by having mercy), therefore, the Rabbis taught that if a pregnant woman is having problems giving birth it is permissible to cut the fetus up because the fetus is like a pursuer who is chasing the woman to kill her, provided, that once the fetus crowns (the head comes out of the uterus), you cannot touch the baby because we do not set aside one life for another (see PRINCIPLE 2) and this (the baby endangering the mother's life) is the nature of the world" - end of quote.
Rabbi Chaim notes that many have questioned this quote based on the Talmud - Sanhedrin 72b, which relates that if a minor (male under 13) is pursuing another minor, you may kill the pursuer even though he is a minor (and exempt from the Torah's prohibitions on murder - for which he forfeits his life under PRINCIPLE 3). If this is the case, that even a minor can be a pursuer, then why are we not permitted to kill the baby who has crowned who is now endangering the life of the mother? The Talmud distinguishes the case of the minor pursuer from the crowned baby pursuer because the crowned baby pursuer is different in that "it is from the heaven that the pursuer is coming" - in other words, the crowning baby is essentially a messenger of the divine will and without any possible intent to kill, so that the baby has not falllen into PRINCIPLE 3.
The Rambam concludes similarly and the crowned baby cannot be touched to save the mother and we enforce PRINCIPLE 2, and just to reinforce this point, we do not label the crowned baby a pursuer that would trigger PRINCIPLE 3.
So now Rabbi Chaim asks why the Rambam wrote in respect of the fetus that we are to kill it because of PRINCIPLE 3, when there is no PRINCIPLE 3 for a crowned baby, so why is the fetus any different. How can Rambam cite to PRINCIPLE 3?
Rabbi Chaim concludes that PRINCIPLE 3 is not relevant to the fetus, and the reason we can kill the fetus is because it is not a "soul" that, if killed, would violate PRINCIPLE 2. Accordingly, PRINCIPLE 1 applies in full force.
But Rabbi Chaim persists in his critique of the Rambam asking why it was necessary to even refer to PRINCIPLE 3? Rambam could have said, you kill the fetus because it is not a soul, therefore PRINCIPLE 2 does not apply, and you apply PRINCIPLE 1. Why refer to the rule of the Pursuer??