Profile of Philip N. Johnson-Laird.

O ur greatest technological accomplishments, from space travel to nuclear power and the creation of the Internet, stand as testaments to the scientific process and mankind’s ability to reason. These advances, however, have been matched with equally spectacular technological catastrophes. “Scientists and technologists are rational in principle,” explains Philip Johnson-Laird, a professor of psychology at Princeton University and recently elected member of the National Academy of Sciences. “But the more information they have to take into account, the more working memory they need and the longer time it takes them to make an inference.” When complex technology starts spiraling out of hand, this abundance of information hinders our ability to make reliable decisions. “Eventually,” Johnson-Laird says, “the computational demands overwhelm them, and this often culminates in catastrophes.” Johnson-Laird has been studying the human ability to reason for nearly a half a century. His theory of mental models, which outlines how real or imaginary situations are represented in the brain, has challenged long-held assumptions about how humans think. “Mental model theory implies that we are rational because we grasp that an inference is no good if we can think of a counterexample to it,” he says. “In practice, however, we often err, failing to find, search for, or heed a counterexample.”