The nature of consciousness seems to be unique among scientific puzzles. Not only do neuroscientists have no fundamental explanation for how it arises from physical states of the brain, we are not even sure whether we ever will. Astronomers wonder what dark matter is, geologists seek the origins of life, and biologists try to understand cancer—all difficult problems, of course, yet at least we have some idea of how to go about investigating them and rough conceptions of what their solutions could look like. Our first-person experience, on the other hand, lies beyond the traditional methods of science. Following the philosopher David Chalmers, we call it the hard problem of consciousness.
But perhaps consciousness is not uniquely troublesome. Going back to Gottfried Leibniz and Immanuel Kant, philosophers of science have struggled with a lesser known, but equally hard, problem of matter. What is physical matter in and of itself, behind the mathematical structure described by physics? This problem, too, seems to lie beyond the traditional methods of science, because all we can observe is what matter does, not what it is in itself—the “software” of the universe but not its ultimate “hardware.” On the surface, these problems seem entirely separate. But a closer look reveals that they might be deeply connected.
Consciousness is a multifaceted phenomenon, but subjective experience is its most puzzling aspect. Our brains do not merely seem to gather and process information. They do not merely undergo biochemical processes. Rather, they create a vivid series of feelings and experiences, such as seeing red, feeling hungry, or being baffled about philosophy. There is something that it’s like to be you, and no one else can ever know that as directly as you do.
Our own consciousness involves a complex array of sensations, emotions, desires, and thoughts. But, in principle, conscious experiences may be very simple. An animal that feels an immediate pain or an instinctive urge or desire, even without reflecting on it, would also be conscious. Our own consciousness is also usually consciousness of something—it involves awareness or contemplation of things in the world, abstract ideas, or the self. But someone who is dreaming an incoherent dream or hallucinating wildly would still be conscious in the sense of having some kind of subjective experience, even though they are not conscious of anything in particular.
Philosophers and neuroscientists often assume that consciousness is like software, whereas the brain is like hardware.
Where does consciousness—in this most general sense—come from? Modern science has given us good reason to believe that our consciousness is rooted in the physics and chemistry of the brain, as opposed to anything immaterial or transcendental. In order to get a conscious system, all we need is physical matter. Put it together in the right way, as in the brain, and consciousness will appear. But how and why can consciousness result merely from putting together non-conscious matter in certain complex ways?
This problem is distinctively hard because its solution cannot be determined by means of experiment and observation alone. Through increasingly sophisticated experiments and advanced neuroimaging technology, neuroscience is giving us better and better maps of what kinds of conscious experiences depend on what kinds of physical brain states. Neuroscience might also eventually be able to tell us what all of our conscious brain states have in common: for example, that they have high levels of integrated information (per Giulio Tononi’s Integrated Information Theory), that they broadcast a message in the brain (per Bernard Baars’ Global Workspace Theory), or that they generate 40-hertz oscillations (per an early proposal by Francis Crick and Christof Koch). But in all these theories, the hard problem remains. How and why does a system that integrates information, broadcasts a message, or oscillates at 40 hertz feel pain or delight? The appearance of consciousness from mere physical complexity seems equally mysterious no matter what precise form the complexity takes.
Nor would it seem to help to discover the concrete biochemical, and ultimately physical, details that underlie this complexity. No matter how precisely we could specify the mechanisms underlying, for example, the perception and recognition of tomatoes, we could still ask: Why is this process accompanied by the subjective experience of red, or any experience at all? Why couldn’t we have just the physical process, but no consciousness?
Other natural phenomena, from dark matter to life, as puzzling as they may be, don’t seem nearly as intractable. In principle, we can see that understanding them is fundamentally a matter of gathering more physical detail: building better telescopes and other instruments, designing better experiments, or noticing new laws and patterns in the data we already have. If we were somehow granted knowledge of every physical detail and pattern in the universe, we would not expect these problems to persist. They would dissolve in the same way the problem of heritability dissolved upon the discovery of the physical details of DNA. But the hard problem of consciousness would seem to persist even given knowledge of every imaginable kind of physical detail.