She nodded. She put the letters in her bag, which she fastened with a snap of the clasp. I lit up a second cigarette and ordered another whiskey. The second whiskey is always my favorite. From the third on, it no longer has any taste. It's just something to pour into your stomach.
In other words, du/dw = u' > 0 and u" < 0. The law of diminishing marginal utility, q.e.d.