The central argument is about the shifting burden of proof for active investors. In the hunt for opportunities to outperform the market, we often assume the key is seeing things as they are: that we have the relevant information, are free of bias, have made reasonable estimates of future cash flows, and so on. Mauboussin’s broad argument is that you can’t make money by just being right. You have to understand why the people “on the other side” of your trade are wrong. What bias, factual error, analytical mistake, or structural market trap they have fallen into. You must be able to “articulate why an inefficiency exists and how it will be extinguished.” It’s not enough to build a net present value model that spits out a number higher than the current price.