I'm not aware of any philosophical uses of this phenomenon. I myself would be inclined to think that unless we can show that these experiences are veridical (in other words, if by some scientific process, we could show that those who experience deja vu actually were "there before"), we should not count them as evidence for anything other than the (obviously true) claim that many human intuitions and experiences can be highly unreliable, and so we should be extremely cautious about which of these we allow to count as evidence for or against anything.

The answer to your question will depend on what counts as a fact.

Certainly it is possible for people to observe the same state of affairs while reacting to it differently. You observe the same wallpaper as I do, but you like it while I hate it. Or we both observe the same behavior on the part of a friend, but you find it wierd while I find it normal.

On the other hand, if the likability of the wallpaper is itself a fact that you (alledgedly) observe, then you and I do disagree about the facts when I find it hateful. Likewise, if the wierdness of a friend's behavior counts as a fact.

Rather than argue over what counts as a fact, it is probably best to recognize that different people will focus on different facts (without disputing the facts that others focus on), and that different people will value the same facts in different ways (without disputing others' descriptions of what they value). This does not mean that our focus of attention or our valuation is purely subjective, or that attention and valuation are not themselves matters for rational dispute; it only means that it is often useful to distinguish between the facts that people agree on and the ways in which they approach these facts differently.

Compare the case of pain to the heat of spicy curries or steamy saunas: I recognize certain green curries to be just as hot/spicy as certain red curries, but I have a higher tolerance for the heat/spicyness of green curries; and I can tolerate steamy saunas better than in dry saunas even when I experience them as equally hot. I am not less sensitive to the heat of green curries or the heat of a steamy sauna, but I am not bothered by them as much as I am bothered by the heat of spicy curries or dry saunas. Why isn't pain like this -- tolerated differently in different forms, or by different people, even when the amount or degree of pain is recognized to be the same?

You might think that pain just is intolerance, and that the degree of one's pain is equivalent to the degree of one's intolerance, so that finding a sauna less intolerable should be equated with finding it less painful. But since the two words, "pain" and "intolerance" are used in rather different ways, and since (as you note) people do often say they are more or less tolerant of pain, it would not a linguistic equivalence. Still, as you suggest, if the evidence for one is equally evidence of the other then perhaps they are in fact the very same thing.

I do not think the evidence for one is the same as the evidence for the other, however. Aside from introspective reports (which constitute one kind of evidence, after all), here are some (more objective) kinds of evidence that help to distinguish insensitivity to pain from tolerance of pain:

(1) A racing heartbeat and tensed muscles (showing sensitivity to pain) together with a calm demeanor (showing tolerance of pain).

(2) A spontaneous cry or flinch (showing sensitivity to pain) together with willingness to continue painful procedure or activity (showing tolerance of pain).

(3) An ability to give nuanced reports on the character of one's pain (showing sensitivity to pain) together with lack of interest in using pain medications (showing tolerance of pain).

While (1) and (2) may suggest two parts oneself -- one responding automatically and the other more reflectively resists these automatic responses, (3) suggests an interesting, if unusual, disconnect between the feeling of pain and the awfulness of pain. You may be interested to look at a recent book by Nikola Grahek, entitled Feeling Pain and Being in Pain, that describe some carefully documented cases of pain without painfulness and painfulness without pain.

It isn't clear to me exactly what sort of academic "topic" interests you the most. So, I'll consider several options.

First, consider whether some academic disciplines are more difficult than others. Given the sheer diversity of academic work within each of the various disciplines, I don't think there is any prospect of ordering those disciplines by difficulty: each field addresses a multitude of problems in a multitude of ways all the while responding to past work in more or less sophisticated ways and also developing methodological innovations, incorporating new content, etc. So, I think that professionals working within every field have opportunities to perform extremely challenging academic work.

Second, consider whether some academic courses are inherently more difficult than others. It certainly is the case that undergraduate students frequently consider some classes to be more difficult than others. In part, this may be reasonable: for example, many students find research methodology and statistics to be among the most challenging courses. Even with respect to individual undergraduate courses, however, it is certainly possible for the instructor of any course to have high and challenging expectations for his or her students and it is certainly possible for any student to choose to increase his or her level of effort, engagement, and intellectual depth. So, I think both students and professors have opportunities to make any academic course challenging.

Third, consider whether some specific learning activities or course topics are more difficult than others. Surely different students find various activities or topics more challenging than others, but here too I think there is significant variation that prevents us from drawing general conclusions of the sort I think interest you: students' individual levels of academic preparation and skills will cause some activities or topics to be harder for them to complete or master; students' individual intellectual interests will make it easier or harder for them to engage effectively in different learning activities or academic topics; students' varying responsibilities or life experiences at various points in their academic careers will make it easier or harder for them to learn at different times, etc.

So, I see little prospect of reaching judgments about "objective difficulty" at the level of disciplines, classes, or learning activities or course topics.

I know several people who believe such things, or at least say they do.

One group thinks that there are true contradictions that involve very special cases. The usual example is the so-called liar sentence, "This very sentence is not true". There is a simple argument that the liar sentence is both true and not true, and some people believe just that.

Other people, though, think there are contradictions involving much less special cases. An example would be what are called "borderline cases" of vague predicates, like "bald". People often want to say that there are some people who aren't bald and aren't not bald either. But the so-called De Morgan equivalences entail that this is equivalent to saying that the person is both bald and not-bald (or, strictly, both not-bald and not-not-bald).

People who hold such views are known as "dialetheists". See this article for more.

You might want to take a look at some of the recent work of John Searle, such as The Rediscovery of the Mind. Searle argues there that "The notion of an unconscious mental states implies accessibility to consciousness. We have no notion of the unconscious except as that which is potentially conscious." (p. 152). While this isn't quite an endorsement of what you call the "transparency thesis," I believe it might be seen as a quasi-descendant of the view you describe.

As an aside, the name "the transparency thesis" is nowadays often used by philosophers of mind to refer to quite a different phenomenon, namely, that when we introspect our experience, we don't seem to be able to attend directly to it; rather, our attention seems to slide right through to the objects of our experience.

It is reasonably unclear what "mental" and "mind" mean overall, though it is clear that whatever it is the intellect is a central part of it, and so if some philosophers want to insist that the mind is to be taken as the conscious mind, there is nothing to stop them, and there is a reasonable point to their stipulation. But even as early as the turn of the 17c, Leibniz accepted unconscious thinking, and there is a very strong case to answer here. Not answering it can make the stipulation look very arbitrary.

Locke died in 1704, by the way, and Berkeley in 1753, so they did not hold the view that you attribute to them "in the later 1700s". And Hume's philosophical writing was done early on in his career; the Treatise and the Inquiry were published before 1750.

If I dream that I am in Lisbon, it does not follow that I am, and I may not be. Nor does it follow that I am not, of course. But if I say that I dreamed that I was in Lisbon last night, this may be one way of saying that I was not in Lisbon. If then I say that I dreamed (in a "ground-floor" dream) that I dreamed something (in a second-level dream, so to speak), it seems to follow that I did not; I did not produce a dream within my dream. And it seems to me hard to see how I could. For the contrast is between dreams and reality, and there is no reality for the second-level dreams to return to. There are just sequences of images and dreamed descriptions of them. If I dream that I have a dream, the second thing I have is not a dream within a dream, but just part of the ground-floor dream.

I agree with Allen Stairs comments about the logic and the possibility of dreams within dreams. I wonder, though, whether your question is also about the possibility of different "levels" of reality. When we 'wake up' from one dream into the reality of another, are we shifting from one level of reality to another or are both dreams equally unreal? There is certainly a tradition (in philosophy and in religion) that embraces the idea of different levels of reality, and that often characterizes the move from one level to another as being like awaking from a dream. From any particular standpoint, however, there seems to be a single line that divides what is real from what is unreal. So from my current waking standpoint, the dream within a dream is just as unreal as the dream that contains it.

I agree with Mitch Green's response with regard to the possibility of controlling our beliefs indirectly. But in the case of a jury member instructed to ignore certain evidence, it is not really belief that is at issue. The jury member is asked to reach a conclusion following certain legal guidelines, but there is no requirement that the juror believe that conclusion -- and thus no requirement that the legal guidelines constrain belief. A jury member can wholeheartedly believe that a defendant is guilty yet confidently cast a "not guilty" vote -- because her belief is based on impermissible evidence.

Some philosophers (Jonathan Cohen and Michael Bratman, for example) use the term "acceptance" as opposed to "belief" to refer to an attitude of endorsement that is restricted to certain contexts or concerns. In the case of a jury member, the endorsement is restricted to legal contexts; in the case of an actor, the endorsement is restricted to a theatrical context; and in the case of an athlete, the endorsement might be be restricted to certain performance contexts.