What does "at that point the age-old question of the good life comes into play" mean?
Human beings are born with more instincts than any other animal on Earth. Among these instincts are two fears: fear of falling and fear of loud noises. Apparently, when a doctor slaps a newborn to get it to breathe, it is the sound of the slap, not the pain of the slap, that startles the infant. Its pain centers are not developed at the age of a few seconds old; however, it can hear perfectly. Also, it is implied that the doctor could hold the child up and drop it a foot or two, which would scare it and cause it to gasp its first breath. Oddly enough, we never lose either of those two fears, so sound pressure (commonly called volume) remains a valuable tool for getting someone’s attention.