From the aricle: "CARE is a noble organization that fights starvation. It would like your support. The American Cancer Society is a noble organization that fights disease. It would like your support, too. Here's my advice: If you're feeling very charitable, give generously—but don't give to both of them. ... Giving to either agency is a choice attached to a clear moral judgment. When you give $100 to CARE, you assert that CARE is worthier than the cancer society. Having made that judgment, you are morally bound to apply it to your next $100 donation."

Landsburg is wrong, and here's why. Because the world is shades of gray, not black and white. It's not clear what the best charity is, even by one's own standards (partly because those standards are not clear, and they sometimes conflict with each other). We know ourselves well enough to know we're not smart enough to make those judgments perfectly, so we don't bother with trying for perfection, but rather with making sure to do at least some good. It's hedging our bets knowing that some of the money is going to the "wrong" charity (we're just not sure which one is "wrong").