Often, I asked myself, How would Greg think? I adopted his habit of tracking what I knew and how well I knew it, so that I could separate my well-founded opinions from my provisional views. Bad investors, Greg told me, often had flat, loosely drawn maps of their own knowledge, but good ones were careful cartographers, distinguishing between settled, surveyed, and unexplored territories. Through all this, our lives unfolded. Around the time I left my grad program to try out journalism, Greg swooned over his girlfriend’s rational mind, married her, and became a director at a hedge fund. His net worth is now several thousand times my own.