Here is something you are doing right now that is, when you think about it, deeply strange. As you read these words, there is something it is like to be you. The black marks on a screen look a certain way, there is a visual quality to them. Your chair feels a certain way against your body. There may be sounds in your environment that have a particular audible character. There is, in short, a subjective dimension to your experience, a felt quality, an inside view.
Now consider your laptop. It is also processing information right now, converting electrical signals, executing instructions, rendering this page. But there is presumably nothing it is like to be your laptop. There is no inside view. The lights are on in you; in the laptop, there is no one home.
What is the difference? This is the central puzzle of the philosophy of mind. It is not a puzzle about information processing, we have a reasonable account of how brains process information, and cognitive science is making progress on it. It is a puzzle about why information processing in one kind of physical system (biological brains) is accompanied by subjective experience while information processing in another kind (silicon chips) apparently is not.
David Chalmers named this the hard problem of consciousness in a famous 1995 paper. He distinguished it sharply from what he calls the easy problems, explaining how the brain discriminates stimuli, integrates information, reports mental states, focuses attention. These are "easy" not because they are simple but because they are the right kind of problem: if you fully explained the underlying functional and neural mechanisms, you would have explained the phenomenon. But consciousness is different. Even if you gave a complete functional and neural account of how the brain processes a red rose, the wavelengths of light, the retinal responses, the neural pathways, the behavioral outputs, you still would not have explained why there is something it is like to see red. Why does the processing feel like anything at all?
The terminology comes from Thomas Nagel's 1974 paper "What Is It Like to Be a Bat?" Bats navigate by echolocation. We can describe the physics of echolocation, the neural processing, the behavioral repertoire. But can we know what it is like, from the inside, to experience the world through echolocation? Nagel says no. Subjective experience has a first-person character that cannot be fully captured by any third-person scientific description, no matter how complete.
Think of the analogy of explaining a color to someone who has been blind from birth. You can give them the full physics of red light, wavelength 700 nanometers, frequency 430 terahertz. You can describe the retinal cone cells it activates. You can trace the neural pathway from retina to visual cortex. You can describe every behavioral consequence of seeing red. But none of this tells them what red looks like. There is a fact about redness that your description, however complete, does not capture. That irreducible fact is what the hard problem is about.
Quick reflection
Imagine a complete neuroscientific account of what happens in your brain when you taste coffee — every neural firing, every chemical process, every functional role. Would that account fully explain the experience of tasting coffee? If you think something is left out, what is it — and why does it resist scientific explanation?