The researchers found, first, that the rate of detection for the swaps was shockingly low—of 354 manipulated trials, only 46—or 13 percent—were immediately discovered by the volunteers. Second, the subjects were no better at remembering choices between dissimilar faces than they were at recalling choices between faces that resembled one another. Finally, when asked to explain their falsified choices, the subjects used a range of confabulations that not only echoed the reasoning they used to support their “real” choices but in some cases involved rejecting features of their actual selections. For example, after having picked a photo of a woman with no visible jewelry, one male subject was then given a picture of a woman wearing very large hoop earrings. He promptly explained that he had chosen as he had because “I like earrings!” Perhaps even stranger were the confabulations that involved attributing a feature to the manipulated image that was actually present only in the originally chosen picture. One female subject declared that she had selected a picture of an unsmiling woman “because she was smiling”—as the subject’s first choice had in fact been. Subsequent experiments have demonstrated that people are generally unable to detect reversals of their own prior stated choices with respect to the taste of jam, the smell of tea, abstract art patterns, monetary gambles, and even political/moral positions. They are perfectly willing to offer elaborate stories to support choices they didn’t make, and to criticize choices they made. Indeed, they are happy to offer ex post facto explanations for just about anything. Which returns us to one of the questions at the heart of “The Road Not Taken”: If we don’t understand why we made a decision—if we don’t know why we “took the other, as just as fair”—then how can our choices be meaningful?

THE DEBATE OVER free will and determinism is ancient. The Greeks first framed it; as Leucippus remarked in the fifth century B.C., “Nothing comes to be at random, but everything for a reason and by necessity,” which leaves little room for human agency. And Epictetus asked, “How do events happen? They happen as the Disposer of events has ordained them,” thereby giving us the essence of several hundred years of arguments over divine foreknowledge. But for modern readers, the most relevant early description of determinism comes from the Stoic philosopher Chrysippus:

Everything that happens is followed by something else which depends on it by causal necessity. Likewise, everything that happens is preceded by something with which it is causally connected. For nothing exists or has come into being in the cosmos without a cause.