Prompted by Maniakes', but sufficiently different to post separately:

It cannot have escaped philosophers' attention that our fellow academics in other fields--especially in the sciences--often have difficulty suppressing their incredulous amusement when such topics as Twin Earth, Swampman, and Blockheads are posed for apparently serious consideration. Are the scientists just being philistines, betraying their tin ears for the subtleties of philosophical investigation, or have the philosophers who indulge in these exercises lost their grip on reality?

These bizarre examples all attempt to prove one "conceptual" point or another by deliberately reducing something underappreciated to zero, so that What Really Counts can shine through. Blockheads hold peripheral behavior constant and reduce internal structural details (and--what comes to the same thing--intervening internal processes) close to zero, and provoke the intuition that then there would be no mind there; internal structure Really Counts. Manthra is more or less the mirror-image; it keeps internal processes constant and reduces control of peripheral behavior to zero, showing, presumably, that external behavior Really Doesn't Count. Swampman keeps both future peripheral dispositions and internal states constant and reduces "history" to zero. Twin Earth sets internal similarity to maximum, so that external context can be demonstrated to be responsible for whatever our intuitions tell us. Thus these thought experiments mimic empirical experiments in their design, attempting to isolate a crucial interaction between variables by holding other variables constant. In the past I have often noted that a problem with such experiments is that the dependent variable is "intuition"--they are intuition pumps--and the contribution of imagination in the generation of intuitions is harder to control than philosophers have usually acknowledged.

But there is also a deeper problem with them. It is child's play to dream up further such examples to "prove" further conceptual points. Suppose a cow gave birth to something that was atom-for-atom indiscernible from a shark. Would it be a shark? What is the truth-maker for sharkhood? If you posed that question to a biologist, the charitable reaction would be that you were making a labored attempt at a joke. Suppose an evil demon could make water turn solid at room temperature by smiling at it; would demon-water be ice? Too silly a hypothesis to deserve a response. All such intuition pumps depend on the distinction spelled out by McLaughlin and O'Leary-Hawthorne between "conceptual" and "reductive" answers to the big questions. What I hadn't sufficiently appreciated in my earlier forthright response to Jackson is that when one says that the truth-maker question requires a conceptual answer, one means an answer that holds not just in our world, or all nomologically possible worlds, but in all logically possible worlds. Smiling demons, cow-sharks, Blockheads, and Swampmen are all, some philosophers think, logically possible, even if they are not nomologically possible, and these philosophers think this is important. I do not. Why should the truth-maker question cast its net this wide? Because, I gather, otherwise its answer doesn't tell us about the essence of the topic in question. But who believes in real essences of this sort nowadays? Not I.

Daniel Dennett, "Get Real" (emphasis added).