Mental models are imperfect, but useful. There is no single mental model from physics or engineering, for example, that provides a flawless explanation of the entire universe, but the best mental models from those disciplines have allowed us to build bridges and roads, develop new technologies, and even travel to outer space. As historian Yuval Noah Harari puts it, “Scientists generally agree that no theory is 100 percent correct. Thus, the real test of knowledge is not truth, but utility.”