Everybody in Spain knows the play Life is a Dream (La Vida es Sueño). It is required reading for Spanish high school students, just as Shakespeare’s Hamlet is commonly assigned in the US. The similarity doesn’t end there: Pedro Calderon de la Barca’s plays were written just one generation after Shakespeare’s—and both playwrights explored similar themes. They both sought favour from the lower classes—by poking rich people in the eye—using not-so-subtle ironic commentary to suggest that noble heritage in the ruling class does not necessarily result in noble behavior. Both authors also explored questions that we all ask ourselves, sooner or later, concerning the nature of humanity, like whether our actions are the result of fate or free will. But one thing Calderon de la Barca did better than previous writers, was to explore this specific question: Is life a dream? The philosophical quandary continues to ring true, even now.

The 1630s wasn’t the first time somebody wondered about what’s real versus imagined. Plato’s Allegory of the Cave, written over 2000 years earlier, set the framework. Here Plato presented Socrates’s the following thought experiment: if you were a prisoner who grew up chained within a cave, and the only thing you ever saw was a series of shadow-puppet shows cast onto the opposite wall—performed by hidden puppeteers manipulating their puppets in front of a fire—then, to you, the shadow puppets would be reality. How could you know anything else? This garden path leads inevitably to the conclusion that all of us are those prisoners in the cave, though our caves are made of flesh and bone (our skulls). Our sensory systems, it follows, perceive reality so poorly that our experience is little better than that of impoverished shadows cast onto a wall. And wonderfully, that’s all very true from a neuroscientific standpoint. Scientists have shown, over and over, that the information our sensory systems provide to our consciousness about the world is, in general, wildly inaccurate. Instead, we live our lives in a virtual reality that is only imperfectly based on the physical reality of the world. The entire point of studying illusions is not that they are the cute little errors that the visual system makes, but that they are the only way we can determine how the brain creates the reality that we live in, within the caves of our minds.

This can be a little hard to accept for readers who may not have considered these ideas previously, and who thus believe that their brains are immune to illusions; they may still naively believe that they perceive physical reality accurately. Allow me to correct this impossibility with an example of how illusory perception manifests itself as part of our everyday lives. Consider a professional photographer. This is a person whose job it is to capture images from the world through a camera’s lens, to store for posterity. There is, of course, an art to choosing where and when, and from what perspective, to shoot, and this cannot be diminished in importance. But after the composition is considered, the physical reality of their job is to point a lens at a scene and push a button. Right? Wait a minute, not so fast. They also need to consider the way that their device physically interacts with light. They must take into account the speed of the film (or the corresponding properties of the light-sensing device in their camera), and thus the amount of light in the scene. Otherwise the image will not be captured properly. So, in a very real sense, a photographer is a professional light measurer, who sets the f-stop (light aperture) on the camera, before pointing and shooting. Luckily, the photographer is outfitted with the most sophisticated imaging device in the universe: the human eye. That means they can just look at the scene and instantly know how many photons are reflecting from the surfaces, to set their cameras’ light sensitivity, right? Wrong. Humans are virtually incapable of accurately determining the amount of light in a scene. That’s why every professional photographer you’ve ever seen carries a $50 light meter in their pocket, and uses it frequently. Because that cheap physical device is far more accurate than their priceless ocular apparatus, no matter how much professional training they’ve had. When it comes to measuring the amount of photons in a scene, your visual system sucks. And if it cannot accurately measure something so basic as the amount of light in a scene, then it’s fair to conclude that little of the information in our visual perception is accurate. You can generalize this idea to any number of things that don’t make sense in your life. Like, for example, if your eyes are so wonderful at measuring the sizes of things, why do rulers exist? Are you telling me that a device that can see the width of a human hair needs a stick with paint on it to tell whether an object is 10 versus 11 centimeters wide? Yes, that’s exactly the case, because our vision is incredibly precise but terribly inaccurate. We see everything as a vast array of differences and contrasts, and we use neural guesstimates and algorithms to navigate the world, rather than physical measurements. Your visual system is a bag of tricks that both uses illusions, and works around illusions, to help you survive.

Calderon de la Barca takes this topic into the realm of dreams. I’ll briefly summarize one of the two plays (Calderon’s Two Dreams, by The Magis Theatre Company) performed through February 27th at the Ellen Stewart Theatre in New York City’s East Village, (https://web.ovationtix.com/trs/cal/42). King Basilio of Poland, whose wife died in childbirth, was informed, by the stars (fate!), that his new baby son would grow to be the scourge of his kingdom. So Basilio locked up the boy—named Segismundo—in a tower under lock and key, where he grew to be a young adult unaware that he was, in fact, the prince of Poland. But Basilio wanted to believe that the stars might have been wrong. Perhaps the prince could take control of his own destiny (free will!) and be released after all. But Basilio had to test Segismundo first, to see if he really was a danger to the kingdom. So he had the prince drugged, and placed into the palace, so that when he awoke, the servants would all tell him that he was the Sovereign of Poland. They would all see whether Segismundo acted like a prince or a beast, under controlled experimental conditions. Segismundo failed the test. Within minutes he fell into a rage and killed a treasured servant by defenestration. The king, heartbroken, had Segismundo drugged once again and replaced in the tower. Tragedy ensued, but the critical point now is that Segismundo, upon awaking up in prison, was informed by his keeper that the whole palace episode had not happened for real; it was just a dream. The question is, how could Segismundo know any different?

Indeed, how can any of us know any different? Maybe you are dreaming this article up right now. Maybe you only dreamed your whole life. How could you know? La Vida es Sueño  ends with Segismundo restored to his position as royal heir of Poland, now a dutiful son to his father the king and a kind master to his servants. But as Segismundo considered his newfound fortune, and the theater audience broke into applause, he did not seem all that certain that he might not yet wake up a lifelong prisoner, baffled and alone in his remote tower.