WE ALSO, one might say, create a deception. But this seems a little harsh. Better, maybe, to say that we create a narrative that works for us, and that we can live with. It’s interesting, in light of this, to think about the phenomenon scientists call confabulation. Confabulation isn’t lying, as you might suppose, but rather the invention of explanations or stories on the basis of information that is incomplete, incorrect, or manipulated (as in an experiment, for instance). A person who is confabulating may be saying something thoroughly ridiculous, but he doesn’t actually know that it’s absurd. He’s simply expressing the best available conception of the world as it seems to him, and he believes in that conception unswervingly. “Confabulation,” writes the philosopher William Hirstein, “. . . is a sort of pathological certainty about ill-grounded thoughts and utterances.”

Of course, certainty about “the world as it seems to him or her” will seem especially strange if the person in question has suffered some kind of neurological damage. This is why many of the best-known studies of confabulation involve a parade of unfortunate medical conditions like Korsakoff’s syndrome (sufferers have lost short-term autobiographical memory), anosognosia (in which a paralyzed person appears unaware of his condition), asomatognosia (the sufferer denies ownership of his own body parts), and the legendarily bizarre Capgras syndrome (in which a person believes friends and loved ones have been replaced with physically identical impostors). In each of these cases the patient will, with perfect confidence, provide explanations for various occurrences that are, on their face, impossible—for instance, a person suffering from Korsakoff’s might, if asked what he did yesterday, answer by reciting events that occurred to him twenty years ago.

But where Frost and “The Road Not Taken” are concerned, the most salient examples of confabulation come from so-called split-brain research. Normally, the hemispheres of the brain are connected by a band of neural fibers called the corpus callosum, the function of which is to facilitate communication between the two halves. The corpus callosum of a person with a split brain has been severed, such that the hemispheres can no longer exchange information as they normally would—in effect, the person now has two brains. In a famous series of experiments from the late seventies, the neuropsychologists Michael Gazzaniga and Joseph LeDoux helped show that this condition can lead to confabulation so spectacular that it seems to say something essential and unsettling about the way we handle choices.