Benjamin Franklin, in a 1779 letter to a friend in France:
When I was a child of seven years old, my friends, on a holiday, filled my pocket with coppers. I went directly to a shop where they sold toys for children; and being charmed with the sound of a whistle, that I met by the way in the hands of another boy, I voluntarily offered and gave all my money for one. I then came home, and went whistling all over the house, much pleased with my whistle, but disturbing all the family. My brothers, and sisters, and cousins, understanding the bargain I had made, told me I had given four times as much for it as it was worth; put me in mind what good things I might have bought with the rest of the money; and laughed at me so much for my folly, that I cried with vexation; and the reflection gave me more chagrin than the whistle gave me pleasure.
This, however, was afterwards of use to me, the impression continuing on my mind; so that often, when I was tempted to buy some unnecessary thing, I said to myself, Don’t give too much for the whistle; and I saved my money.
As I grew up, came into the world, and observed the actions of men, I thought I met with many, very many, who gave too much for the whistle.
This should be mandatory reading – preferably re-read to the point of memorization – for anyone thinking about starting a Ph.D. program.