3 ideas
In the title story, Carver, whose wife Amy was killed during a bank robbery, buys a temporary implant that suspends his belief in the sanctity of human life so he can kill her killer. The implant expires after three days, and afterwards, while still feeling moral revulsion at what he did, Carver goes looking for further, more permanent modification.
In "Learning to Be Me," people carry an Ndoli jewel that learns for decades to mirror the brain, and in adulthood the organic brain is removed and the jewel takes over the body. The narrator discovers that he is the jewel, and that he has been diverging from the brain he is meant to copy. He survives the switch, and the ending suggests that hospital staff detect such divergences and hide them by destroying the organic brain.
In "Closer," two lovers use technology to make their minds temporarily identical, hoping for perfect intimacy. Once they separate, they find that total sameness erased the difference that made each one an object of the other's love. Intimacy depends on an irreducible otherness, and eliminating the gap between two selves removes the relationship instead of perfecting it.