Firstly, Frank Jackson introduces us to the case of Fred. Fred can see two colours in a given areas; whereas we can see only one. The problem is that he's unsuccessful when it comes to teaching the rest of us the difference between red1 and red2. We can't make that distinction. Fred, therefore, concludes that “the rest of the world is red1 – red2 colour-blind”. The point is that Fred has a different phenomenal experience than the rest of us. He has the “ability” to distinguish two shades of red which we can't distinguish.
We can, initially, accept this hypothesis. Jackson concludes that it won't help us to understand the difference between red1 and red2 even if we were to know everything there is to know, physically, about both red1 and red2. There must therefore be a phenomenal gap between the physical and either red1 or red2 (perhaps both). Either one or both must run free (as it were) of any physical underpinning. Despite all that, researchers do find a physical explanation as to why Fred can distinguish between red1 and red2. In this hypothetical situation we
“find out that Fred’s cones respond differentially to certain light waves in the red section of the spectrum that make no difference to ours (or perhaps he has an extra cone) and that this leads in Fred to a wider range of those brain states responsible for visual discriminatory behaviour”. 
Jackson then brings on board a modal argument; as well as giving attention to the possibility of zombies. In fact the zombie-possibility starts of with a logical argument - or at least a statement about the limits of logical entailment.
Mary may well have all the physical information about red or about red’s physical underpinnings. Similarly, we may have all the physical information about what seems to be another person. Jackson argues that no
“amount of physical information about another logically entails that he or she is conscious or feels anything at all”.
“But then what is it that we have and they lack?”
“Not anything physical…”
“[c]onsequently there is more to us than the purely physical. Thus Physicalism is false.”
“sincerely deny that there cannot be physical replicas of us in other possible worlds which nevertheless lack consciousness”.
Perhaps we can say here that such beings couldn't exist according to the physical laws of our world. Aren’t philosophers usually talking to us about the possibility of zombies in our world? Or, at the least, about zombies at a possible world which nevertheless still shares our laws of nature? However, isn’t it logically possible that zombies could exist not only at other possible worlds; but also in our own? Is this scenario metaphysically possible?
Tom Nagel offered us something very special to this general debate when he published his paper, ‘What is it like to be a Bat?’ Very generally, S can't tell us what it is like “from a bat’s point of view”. The bat’s point of view is “not our point of view” . In addition, the bat’s point of view
“is not something captureable in physical terms which are essentially terms understandable equally from many points of view”.
All this is pretty much incontestable.
For example, we could work out the physical substructure of another shade of blue by examining it. Though we still couldn't imagine another shade of blue. As for inferring another shade of blue simply from our previous experiences of known shades of blue; this is equally contestable and probably untrue. Indeed both hypotheses seem untrue, at least prima facie. At the phenomenal level only we couldn't do so - according to Nagel (if not Jackson) - because bats, for one, are simply “too unlike us”.
Jackson now introduces epiphenomenalism into the debate.
There are good reasons for holding that qualia are indeed causally inefficacious. For example, “a quale like the hurtfulness of pain must be causally efficacious in the physical world”. A pain is a phenomenal process that can cause us, for example, to remove our hand from the fire. This is a causal relation or link between phenomenal pain and the physical movement of a hand. Surely this causal link is real. However, there's a Humean argument against believing this which makes use of a general position taken from Hume’s well-known stance on causality. Jackson writes:
"No matter how often B follows A, and no matter how initially obvious the causality of the connection seems, the hypothesis that A causes B can be overturned by an overarching theory which shows the two as distinct effects of a common underlying causal process.”
No matter how often the removing of one’s hands follows the person's experience of intense heat, and no matter how initially obvious the causality of that connection seems, the hypothesis that the intense heat causes the removal of the hand can be overturned by an overarching theory which shows the two as distinct effects of a common underlying causal process.
“We may assume that qualia evolved over time… and so we should expect qualia to be conducive to survival. The objection is that they could hardly help us to survive if they do nothing to the physical world.”
“Having a heavy coat is an unavoidable concomitant of having a warm coat… and the advantages for survival of having a warm coat outweighed the disadvantages of having a heavy one.”
“… how can a person’s behaviour provide any reason for believing he has qualia like mine, or indeed any qualia at all, unless this behaviour can be regarded as the outcome of the qualia.”
Jackson then reiterates the basic epiphenomenalist position on qualia. He writes:
“Now the epiphenomenalist allows that qualia are effects of what goes on in the brain. Qualia cause nothing physical but are caused by something physical.”
“Hence the epiphenomenalist can argue from the behaviour of others to the qualia of others by arguing from the behaviour of others back to its causes in the brains of others and out again to their qualia.”
Many, if not all, epiphenomenalists argue that the supposed causal impotence of qualia is a godsend for die-hard (neo) dualists. They merely “sooth” their “intuitions”. The fact remains, however, that they are an "excrescence":
“They do nothing, they explain nothing…”
“it is very likely that there is a part of the whole scheme of things, maybe a big part, which no amount of evolution will ever bring us near to knowledge about or understanding of”.
McGinn, Colin. (1989) 'Can We Solve the Mind-Body Problem?'