What is reality? Is it a game, a simulation that minds beyond our own are skillfully creating for study or for fun? If so, we may be just the characters in their game, foolishly thinking that we are autonomous, sentient beings. Is reality something “out there"—something we simply perceive through our senses? Or is it something “in here"—derivative of the inner workings of the brain, a fabrication, so to speak, of how this small mass of some 85 billion neurons integrates stimuli and creates a sense of the real?
Consider Plato’s allegory of the cave. Slaves, chained from birth so that they could only stare forward, watched shadows dance on the wall—the wall and the shadows were their reality. Plato’s point is that our sensory perception of reality is a farce. Just like the slaves, we see only phantoms. To truly see, he argued, we’d have to break free of the chains of the senses and look bravely at the fire, at the light of pure knowledge. Only within the rarefied domain of pure thought could we hope to ascend toward wisdom. For Plato, truth dwelt in the realm of thought, expressed through the beauty of mathematics and eternal forms. The world “out there” was a big waste of time.
Of course, the world out there is the focus of the physical sciences, and to physicists, at least, it is far from a big waste of time. In fact, up to the early 20th century, the world out there was the only source material for physics. Mechanics, gravity, electromagnetism, the study of heat—these were the pillars of classical physics, a quantitative description of the natural phenomena that we can perceive objectively. When our senses couldn’t catch something—a microbe, a distant galaxy—our tools were there to help. Scientific tools are reality amplifiers, devices that reveal what otherwise would be invisible. To a 19th-century physicist, reality was what could be captured with our senses, properly augmented by tools of exploration.
Surely, though, this is not the whole story. Our tools are limited; they have ranges, and their measurements are always within a certain margin of precision. Even if our tools greatly increase our perception of physical reality, they tell us only part of the story. This is not something that will change. Tools of exploration are dependent on the technology of the time. They may see progressively more, but they can’t see it all. Just as in Plato’s cave, the reality that we collect through our tools is always made of shadows on a cave wall. The wall is our perception of the real.
Things get even hairier as we move to modern physics. Quantum mechanics, the physics of molecules, atoms, and subatomic particles, upset one of our dearest notions in science: objectivity. To see the world, we need to observe it. To observe the world, we need to detach ourselves from it. We see that chair, and we can sit on it. You and the chair are not the same object. However, when we go to the world of the very small, this separation is lost. Without the detachment of classical physics, the notion of reality becomes convoluted: There is no “out there” any more. There are also no electrons or other quantum entities. They are constructions, devices we use to describe the data measured with different sets of equipment. As we dive into the very small, things that are, aren’t for long; they transmute into one another, appear and disappear, bursting into puffs of electromagnetic radiation.
Many of the bizarre effects of quantum mechanics depend on having two or more possible states of a single entity superimposed on one another. For example, the vertical and horizontal polarization of a single photon. Or, if you don’t like cats, a dead cat and a living cat. If you find that two possibilities coexisting in a single state is strange, including a living thing and a dead thing, you are not alone. Einstein, Schrödinger, Planck, and many other eminent physicists hated it. Fortunately, “decoherence” saves the day: Quantum superpositions are very fragile, and interactions with the external environment can easily destroy them, thus restoring sanity, i.e., a more classical way of dealing with the world.
This doesn’t mean, of course, that quantum superpositions don’t exist. They surely do, and are the very core of quantum physics. But there is a separation of scales, where our classical perception of reality takes over. We don’t know exactly where this happens, though. And this is a mystery that speaks directly to the nature of reality. What is an observer? Is it the detector that spots the photon and reads its polarization? Is it the human observer, needed to (a) construct the detector; and (b) make sense of the data? And aren’t detectors and human observers also made of atoms? Aren’t we all quantum things? If so, how many atoms does it take for a detector, machine or human, to become a classical object, decohered from quantum weirdness?
The debate over quantum reality invariably takes us to the human mind. Many physicists casually dismiss the connection as a waste of time, given that the business of physics is to make sense of the data our devices collect, nothing more. But behind the dismissal lies the fear that objectivity is impossible. If we can’t separate observer and observed, then what we call reality becomes what we see of it. That’s unsettling because we feel in our bones that there ought to be more to the world than what we can measure of it. However, if we are stuck with our measurements, we can’t ever truly know the world. All we can do is swim around the bowl, like trapped fish. What we call the world is only what we can see of it. We shouldn’t be calling it the world; only our world.
The consequences are disturbing. We are locked into a restricted view of reality. We live on an island of knowledge surrounded by the ocean of the unknown. And although the island expands as we learn about the world, so do the shores of our ignorance, the boundary between the known and the unknown. We are surrounded by mystery. And that’s a very good thing. Einstein said it well:
The fairest thing we can experience is the mysterious. It is the fundamental emotion that stands at the cradle of true art and true science. He who knows it not and can no longer wonder, no longer feel amazement, is as good as dead. A snuffed-out candle.
“But wait!” someone protests. “We can amplify our perception of reality if we amplify our intelligence. The human brain is just a transient stage in the evolution of intelligence. We are going to become ever more powerful and soon become transhuman. Surely these limitations you speak of won’t apply then.” This is the strong artificial-intelligence position: that it is possible to build machines capable of not just following instructions in a code, but also of creative thought, of consciousness. How realistic is this? Should we worry that our days as creatures of flesh and blood are numbered? Could it be that we will become something else, a machine-human hybrid, capable of understanding and engaging with reality in completely different ways? And if so, are we deliberately pursuing our demise, engaging in engineering our next evolutionary stage?
Responding to the questions are two emerging camps (with many shades of gray in between), which we’ll call the Computationalists and the New Mysterians. Computationalists, like the neuroscientists Henry Markram and Antonio Damasio, and the philosophers Patricia Churchland and John Searle, believe that we can build the human mind from bottom up—gathering detailed information of the minute workings of the brain neuron by neuron, and transferring all that to a huge computer program.
The New Mysterians, which include the philosophers David Chalmers, Colin McGinn, and Thomas Nagel, believe that the project is doomed to fail: To simulate the human mind, we need to understand not only its neuronal structure but also its complete functioning, including, in particular, the nature of consciousness. And that, claim the New Mysterians, is something we humans can’t do. The Computationalists are naïve to assume that something like the human mind will simply emerge from computer simulations, the reasoning goes.
Henry Markram, who heads the billion-euro Human Brain Project in Europe, is betting that supercomputers will reach the exaflop mark by 2018: They will be able to process one million trillion operations per second, equivalent to the loosely estimated power of the human brain.
But is consciousness just a matter of number crunching, evolving algorithms, parallel processing, and access to huge data banks, or is there some fundamental science missing? There is no question that any effort to build a thinking machine by emulating brain function is a worthy scientific project. The question is whether simulating the human mind is a realistic scientific goal. You may argue that we can know only if we try. And try we must. However, as we consider how the mind works, we must realize that there is a fundamental difference between the object of perception (seeing the painting) and the substance of thought (the experience of seeing the painting).
Nagel and McGinn argue that computation alone cannot give rise to consciousness. McGinn points out that the subjective experience of the self is not an empirical quantity amenable to the usual methodology of science, in which a system under study is neatly isolated from its surroundings. We get back to the fish in the bowl, to the binding between observer and what is observed, to the cognitive limitations inherent in being human and thus inherent to our construction of reality.
If mind is all matter, it is not simply reducible to neuronal activity as currently understood. We seem to be missing an essential piece of the puzzle, one that the New Mysterians don’t think we are able to formulate consistently. If they are right, and I think they are, we should find comfort in our failure: We may simulate other kinds of minds, but ours will remain unique.
Reality is something we construct. Even if there is such a thing as an “out there” reality, we have no access to it. What we can see and ponder is predetermined by the embodiment of our cognitive processes. We can use tools to extend our vision of the world, we can use computers to extend our brainpower, but in the same way that a mouse can’t sing opera, much less compose one, we wouldn’t know how to emulate a human mind from scratch.
In a striking way, when it comes to understanding the mind we may invoke the universe as a whole: We can’t step out of it to study it from an objective perspective. All we can do is inch around from the inside, trying to make sense of this or that detail, but forever precluded from embracing the whole. We may hold a brain in our hands; but a mind transcends our grasp.
So what can we say about the nature of reality? The world out there depends on the world in here, of how we think and describe perceived reality through models and approximations. Fortunately, we are not locked into watching shadows dance on a wall. We can create our own projections. Limits to knowledge are not insurmountable obstacles, but triggers that propel our imagination forward. As we imagine and invent, our construction of reality changes. We can thank the laws of nature and the limits of knowledge for making sure that we remain human and fallible. There is no end goal; there can’t be. Any scientific explanation is necessarily limited. The map of what we call reality is an ever-shifting mosaic of ideas.
Marcelo Gleiser is a professor of natural philosophy and of physics and astronomy at Dartmouth College. This essay is adapted from The Island of Knowledge: The Limits of Science and the Search for Meaning, published recently by Basic Books.
