Kahneman proposed an alternate measure that assessed pleasure or pain sampled from moment to moment, and then summed over time. Kahneman called this “experienced” well-being and attached it to a separate "self". He distinguished this from the “remembered” well-being that the polls had attempted to measure. He found that these two measures of happiness diverged. His major discovery was that the remembering self does not care about the duration of a pleasant or unpleasant experience. Rather, it retrospectively rates an experience by the peak (or valley) of the experience, and by the way it ends. Further, the remembering self dominated the patient's ultimate conclusion.
Kahneman demonstrated the principle using two groups of patients undergoing painful colonoscopies. Group A got the normal procedure. Group B, unknowingly received a few extra minutes of less painful discomfort after the end of the examination, i.e., more total discomfort. However, since Group B’s procedure ended less painfully, the patients in this group retrospectively minded the whole affair less.
“Odd as it may seem,” Kahneman writes, “I am my remembering self, and the experiencing self, who does my living, is like a stranger to me.