The Mishna (37a) lists the warnings that the judges gave to witnesses who were about to testify in order to ensure that they will speak the truth in cases of dinei nefashot – capital cases. The judges point out the most basic difference between ordinary testimony and testimony in capital cases. While in money matters money that is taken unfairly can always be returned, when capital punishment is carried out unjustly “his blood and the blood of his descendants will be your responsibility forever.” The source for this idea is the passage that describes how when Kayin killed Hevel (see 4:10) the Torah says that Hevel’s blood – demei, in the plural – was crying out to God.
The Mishna follows this with the oft-quoted statement, that Man was created as a unique individual (as opposed to animals and plants, where the creation story indicates that many of each species were created at the same time) in order to teach that whoever destroys a single soul of Israel is considered as though he had destroyed a complete world; and whoever preserves a single soul of Israel, is considered as though he had preserved a complete world. Furthermore, the Mishna continues, creation of Man also attests to the greatness of God, for when a human being strikes coins from a mold he will always create identical coins, while God created a single Man, yet all of His creations are unique.
The method used for making coins involved first creating blank circles of metal that were then stamped with a hammer blow.
It should be noted that although the text of our Mishna reads “that whoever destroys a single soul of Israel is considered as though he had destroyed a complete world; and whoever preserves a single soul of Israel, is considered as though he had preserved a complete world,” alternative readings in manuscripts and other texts do not include the word “Israel” and teach simply “that whoever destroys a single soul is considered as though he had destroyed a complete world; and whoever preserves a single soul, is considered as though he had preserved a complete world.”
The Gemara on today’s daf develops these ideas, emphasizing the importance of the uniqueness of every individual, with Rabbi Meir, for example, teaching that we find three unique qualities in every person – their voice, their appearance and their thoughts.