During my late childhood and tween years I often puzzled over a difficult Question. It was an especially perplexing one because I couldn't figure out how to articulate it correctly. I'm aware now that this hard-to-define Question was actually a combination of questions, and that the central one had to do with what philosophers of mind term qualia.
If I'd had the right terminology at hand, I might have asked: why is there a sense of subjective, conscious experience? And why do I have such a sense; why is it something that is instantiated in the form of an Self? Does everyone else have it too? This third question bothered me especially. It's difficult to prove that anyone else is having a subjective experience, because such experience is, well, subjective. Making statements about it requires knowledge of what's going on inside another person's head.
And even observing the contents of someone's head isn't enough: one could subject another person to an MRI and still be unable to detect whether any qualia were happening in there.
What if I'd been born into a world of zombies? Or suppose this was a world that is partly inhabited by zombies, and partly inhabited by sentient beings who experience qualia? How could we distinguish one from the other? There seems to be no way to logically rule out the possibility that zombies are moving among us. Indeed, a now-famous thought experiment by David Chalmers invites us to conceive of zombie twin: "someone or something physically identical to me (or to any the conscious being, but lacking conscious experiences altogether."
The creature is molecule for molecule identical to me, and identical in all the low-level properties postulated by a completed physics, but he lacks conscious experience entirely....he will be awake, able to report the contents of his internal states, able to focus attention in various places, and so on. It's just that none of this functioning will be accompanied by any real conscious experience. There will be no phenomenal feel. There is nothing it is like to be a zombie. (The Conscious Mind, 94-95)
Chalmers's purpose wasn't to suggest the existence of a zombie population, but to show the difficulty of explaining qualia in physical terms. If we can conceive of a zombie world, though, what's to prevent our actual world from being populated, to some degree, by zombies? How can we know?
Empathy is one way of establishing the reality of other people's subjective experience; humans generally have the capacity to "understand another person's condition from their perspective," to use Psychology Today's definition of the term. The possibility of zombies probably bothered me more at a younger age because younger people are still building their capacity for empathy, some more quickly than others (I was a slow learner). Very small children don't have the capacity at all; they are unabashedly narcissistic.
It's true that empathy is not always reliable; some people know how to fake and manipulate. In theory, a robot could mimic human body language, facial expressions, and vocal inflections so closely that we'd begin to empathize with it. We sometimes respond to the wrong cues: a dolphin, for example, is not really smiling. Its jaws are just shaped that way.
All that aside, though, empathy is reliable at least some of the time. That gives us enough to work with. We can recognize what others are going through; we can relate their experiences to ours. And empathy is not merely a process by which psychological functions in one person trigger a psychological response in another; a non-empathetic psychopath is capable of responding in this way. Rather, empathy involves the recognition that others are conscious beings. We can infer that not only are they having experiences, but that they have the experience of subjectivity -- we're all pinging the network: "here I am, here I am."
Another clue to the prevalence of qualia is the fact that people have philosophical discussions about the topic. If they weren't common to humans, we would have no reason to engage in such conversations. It would be like having late night talks on the subject of zzkvgt or brgrwatsq. (That can happen, after enough booze).
So, for me, the clincher is that the question has been defined and asked. It means others have wondered about it, intensely enough to figure out how to express it precisely. Whatever qualia actually are -- epiphenomena of the brain, the result of "psychophysical laws," or some kind of non-material property -- we can be sure others have them. Most of us, anyhow. I think.