Rephrase that and it sounds nonsensical: "If you can't outperform the stock market, then how can you be sure of anything?" I think Carnegie was just looking for a glib rationalization for his advice to avoid contradicting people whom you want to like you.
This is our monthly thread for collecting these little gems and pearls of wisdom, rationality-related quotes you've seen recently, or had stored in your quotesfile for ages, and which might be handy to link to in one of our discussions.