In Alex Burne’s paper Some like it HOT he says the following,

So I judge the higher-order thought hypothesis to be a heroic failure. That is particularly unfortunate for me, since it is one of the few reductive accounts of phenomenal consciousness that I can understand.

 Byrne is right that he understands the higher-order thought theory. In fact he is one of the very few philosophers I have read on the subject that has a decent grasp on what the theory actually says and how it works.

So, why then does he judge it a failure?

The present problem is that if the higher-order thought hypothesis is true, higher-order thoughts that one is in a sensory state, and which occur in the right way, must be alone sufficient for phenomenal consciousness. And the question is why this should be thought to represent any kind of advance. Has any of the initial puzzlement surrounding phenomenal consciousness been dispelled?

 This is a particularly dangerous line of attack as he is trying to hit the higher-order theory where it hurts most, that is, in its ability to explain consciousness. Byrne’s basic worry is that being told that there is a higher-order thought around doesn’t help to understand phenomenal consciousness any more than when we began.

He goes on to spell the problem out in more detail. He says,

Rosenthal’s official line is that having a higher-order thought that one is in a mental state is not, strictly speaking, sufficient for that state to be conscious. Visual scientists may tell me that I am having a visual experience, and I may believe them – that is, I may have a higher-order thought that I am having a visual experience. But this would not make the visual experience conscious. So Rosenthal adds in the requirement that the higher order thought arises without the benefit of inference or observation of which the thinker is transitively conscious. But surely it is completely mysterious why a state’s having (or lacking) a certain aetiology should be the extra ingredient that turns it into a state that there is something it’s like to be in. And in any case, once we are allowed to appeal to aetiology, why not do it at the level of sensory states, leaving higher-order thoughts by the wayside? It is the way that a sensory state is brought about, let us propose, that makes it phenomenally conscious. That, I take it, does not help to explain phenomenal consciousness, but it does just as well as the higher-order thought hypothesis. (emphasis added)

It is indeed mysterious why being caused in one way as opposed to another, all by itself, could result in phenomenal consciousness in one case and not the other. But this, I think, is not quite the right way of thinkig about what is going on. Accoring to the transitivity principle a mental state is conscious if I am onscious of myself as being in that state. This gives us a ready answer tothe question ‘why is there something that it is like for you to have a conscious mental state?’ The answer is that I am conscious of myself as being in that state in a subjectively unmediated way. It is not the causal history that is important. It is the way that I am conscious of myself that is doing the work.