As Danny and Amos’s collaborative work—some of which was conducted in Eugene, Oregon—gathered momentum, they began to look for mistakes and biases created by the human mind. For Danny in particular, this period of fruitful collaboration changed his life. When he recalled this time together, he remembered, “I would say something and Amos would understand it. When one of us would say something that was off the wall, the other would search for the virtue in it” (180). Their comfort level with each other provided a foundation for their work. In the subsequent months they developed theories about subjective probability, rules of thumb that the mind resorted to, meaning heuristics and mental models. In essence, their arguments revolved around the idea that “the mind had these mechanisms for making judgments and decisions that were usually useful but also capable of generating serious error” (188).

The more Danny and Amos studied the issue and conducted their thought experiments, the more convinced they were that the human mind plays tricks on itself when making judgments. These tricks stemmed from biases, such as what Danny and Amos called the “recency bias” and the “vividness bias.