We often think of data and information as something abstract and best experienced through charts and graphs. Either that or the stuff written to computer hard drives in a zillion 1s and 0s.
Sometimes we forget that our own senses—sight, smell, touch, taste, and hearing—are a type of data, too—sense data. We pick up something small, round, and red that feels delicate and has an earthy smell to it. All this data runs up to our brains as electrical signals, and our brain lets us know what we're dealing with. In this case, our brain says it's a tomato versus, say, a hacky sack, best for eating and not for kicking.
In Neuromancer, cyberspace is described as a "graphic representation of data abstracted from the banks of every computer in the human system. Unthinkable complexity. Lines of light ranged in the nonspace of the mind, clusters and constellations of data" (3.94). If that sounds confusing, well, it's supposed to. Gradually, we'll come to understand just what that means.
In the novel's beginning, cyberspace's data is represented in the abstract forms we often consider data to take. Everything is geometrical and graphed like an old-school Atari video game. Then things slowly change. Wintermute pulls Case into his world, a world constructed on Case's memories. Everything looks, feels, and smells real even though Case knows it isn't. After all, they're his memories. Finally, Case is pulled into Neuromancer's world, a world not constructed on any of Case's memories but still looks, feels, and smells real none the less.
In Neuromancer's world, Case says, "None of this [is] real, but cold [is] cold" (20.51). Given this statement one might ask, what's the difference? You experience reality as sense data being sent to your brain, right? Well, Case experiences Neuromancer's corner of the matrix the same way, as electrical impulses being sent to his brain. So what separates cyberspace's version of reality from, you know, reality?
If cyberspace seems real enough to be getting on with in Neuromancer, we can't help but wonder if this entire novel is a bit of an allegory for what philosophers call the "Brain in a Vat" thought-experiment. The thought-experiment asks you to picture a brain in a vat linked up to a supercomputer. The supercomputer feeds the brain electrical impulse by wires connected to its neurons (Case's console set up, anyone?). These electrical impulses provide sensory data to the brain to create a reality for the brain. In short, this scenario questions whether one could ever know whether it he or she was experiencing the real world or simply a brain in a vat being feed reality. Do we actually have arms and hands and legs and toes? Or are we just squiggly balls of gray matter?
The short answer is, we don't.
P.S. If all of this is starting to sound really similar to Morpheus's "Dream World" speech in The Matrix, that's because they are really, really similar concepts. Maybe Morpheus read Neuromancer for some pointers on how to discuss this stuff with Neo.
P.P.S. In case you're interested in discussing this further, cyberspace serves as both a setting and an allegory in Neuromancer. Don't forget to check out the "Setting" section for more.