Generally speaking, all of the commandments of the Torah are “pushed aside” in the face of potentially life threatening situations. Therefore, the Mishna on our daf teaches that someone who is ill or pregnant and is in a dangerous situation will be allowed to eat on Yom Kippur, or to eat non-kosher food, if necessary.
There are only three mitzvot that are so severe that a person should give up his life rather than perform the forbidden acts. Those mitzvot are:
- Avoda zara (idol worship)
- Gilui arayot (forbidden sexual relations)
- Shefikhut damim (murder)
According to the Gemara, the sources for the first two mitzvot are Biblical passages. (For the source for avoda zara, see 6:5 which teaches that you must worship God with all of your heart and all of your soul. The source for gilui arayot is 22:26 which compares a forbidden sexual encounter with murder.) According to the Gemara, however, the source for murder being forbidden even at the cost of one’s own life does not need to be a pasuk – it is a sevara – it is simply logical. The logic, as presented by Rava in the Gemara is “Mai hazit didama didakh sumac tefei? Dilma dama dihahu gavra samik tefei!” What makes you think that your blood is redder than your fellow’s? Perhaps his blood is redder than yours!
The Maharil Habib in his Tosafot Yom haKippurim explains this argument as simply meaning that we are unable to weigh the true value of one life against another. Since the whole issue at hand is whether we can “push aside” a mitzva in order to save a life, in this case a life will be lost no matter what, so we cannot allow the forbidden act of murder. It should be noted that this argument works even if we are weighing the value of a single life against that of a group of people. Still the rule of mai hazit would not allow the killing of one person, since the relative value of life cannot be determined by numbers.